  LEVELS—METHODS OF ASCERTAINING—LEVEL TUBES—MANUFACTURE—CURVATURE—SENSITIVENESS-TESTING—READING—CIRCULAR LEVELS—SURVEYORS' LEVELS—Y-LEVEL—PARALLEL PLATES—ADJUSTMENTS OF Y-LEVELS—SUGGESTED IMPROVEMENTS—DUMPY LEVEL—TRIPOD STANDS—ADJUSTMENT OF DUMPY—COLLIMATOR—IMPROVEMENTS IN DUMPY LEVEL—TRIBRACH HEAD—DIAPHRAGMS—CUSHING'S LEVEL—COOKE'S LEVEL—CHEAP FORMS OF LEVEL—HAND LEVELS—TELESCOPIC LEVEL—REFLECTING LEVELS—WATER LEVELS. 166.—A Level Plane is understood technically to be a plane truly tangential to the theoretical spheroidal surface of the earth, as represented by any spot upon the mean surface of the ocean or of still water free from local attraction. The importance of having the means of constructing efficient instruments that can be conveniently employed to obtain the correct relative altitudes of points or stations upon the earth's surface, in relation to such a plane or datum, can scarcely be overrated. Such instruments are not only used for topographical surveys of countries, but also in designing and carrying out public works adapted to the local conditions of natural inclination of the land surface, for railways, drainage, irrigation, canals, water-works, and other constructions. 167.—The force constantly at our command to enable us to ascertain relative altitudes and to form mentally or graphically local level lines on the earth, is that of gravity; and 168.—In taking the level of a liquid surface contained in a vessel, we have, as just stated, to keep this surface free from the disturbing influence of air currents, and to surround the surface with equal conditions of capillary attraction, or to make these conditions equal in the direction in which we desire to ascertain our level. This is found practically to be best performed by means of a sealed glass tube, in which the liquid will by gravitation naturally occupy the lower place, and any air or lighter fluid contained therein the space above this. Fig. 47.—Level tube (bubble). Larger image 169.—Level Tubes, or Bubble Tubes, as they are 170.—The operation of grinding is very much the same as that described for lenses, p. 17. The surface is required to be traversed in every direction longitudinally and transversely, which is effected as far as possible by a twist of the hand alternately to the right and left. The tube should also be frequently taken off and turned end for end. Slight variations of curvature are readily made by differences of pressure of the hand on parts of the tube; and a little coaxing is allowed to get the centre of the tube quick where the tube is to be used for levelling only, and not for measuring small angles, so that in this case the finished tube is slightly parabolical. The finishing touch is produced with wash-emery. The inside should be left smooth but not polished, as the slight roughness of a fine ground surface assists the capillary action by causing better adhesion of the spirit, and gives a quicker run to the bubble. Where the tubes are required of a given radius they are tested frequently, during the grinding, upon the bubble trier, by placing two corks in the ends of the tube, which is nearly filled first with water for rough trial, and then with spirit for final correction. 171.—The Bubble Trier is a bar or bed 12 to 20 inches long, with two extended feet ending in points at one 172.—The Sensitiveness of a Level Tube, the upper curvature and ground surface being equal, depends very much upon the capillary action due to its internal diameter, the larger tube, from the freedom of restraint by capillarity, being the more active. As regards the ultimate settling to gravitation equilibrium, perhaps there is no difference, but small tubes are sluggish and take time to work. The following are about the usual dimensions of the interior of sensitive tubes—8 inches × 1 inch diameter, 7 inches × ·9, 6 inches × ·8, 5 inches × ·7, 4 inches × ·6, 3 inches × ·5, 2½ inches × ·45, 2 inches × ·4, 1½ inches × ·35, 1 inch × ·3. The larger the volume the greater the expansion of liquid with heat; the longer the tube the less torsion it is liable to suffer from sealing, so that if possible, as expansion is a serious defect, it would be better to have short tubes, if these could be sealed without disturbance of curvature. Much shorter tubes are used in America than in Great Britain. 173.—The Curvature of a Level Tube is worked to radius according to the delicacy of the work to be performed with it afterwards. The radii of curvature of different level tubes used for scientific purposes vary from about 30 feet to 1000 feet or more. The radius of any curve may be conveniently measured by the relation of its versed sine to its chord of arc, the chord being the length of the tube. If this is first calculated out, a piece of shellac may be attached by melting it down upon the centre of the edge of a parallel glass straight-edge, to represent by its thickness the versed sine. The spot of shellac may be brought to the exact height required from the versed sine = rad - v(rad2 - (½ cho)2). 174.—The general instruction, however, given to the maker is the distance of run of the bubble that is required to give seconds or minutes of arc; and perhaps this is after all the best test for accuracy of the tube which, like all other articles in glass submitted to the process of grinding, is subject to a certain amount of local error. By this method the local error is discovered by testing with the bubble trier. When the run is given, the radius of the curve of the tube may be found if desired by the use of a common multiplier, as follows, very approximately— Arc equal to radius expressed in minutes, 3437·74677. " " " seconds, 206264·80625. The run of a good sensitive tube is frequently made 1/30 inch to the second, here (omitting decimals)— arc sec (206264·8) × 1/30 inch = 573 feet radius nearly. For scientific purposes a millimetre run per second is commonly used, then— arc sec (206264·8) × ·001 metre = 206·264 metres radius or 680 feet nearly. For an ordinary 12-inch dumpy level the tube is divided into minutes at about 1/10 inch apart, radius 28 to 30 feet; for a sensitive 14-inch Y-level of good construction the same divisions may represent five seconds, radius of bubble tube about 170 feet. 175.—The Divisions upon Ordinary Level Tubes are made after the tube is finished, but with the highly sensitive ones the divisions are made first. The run is taken from 176.—Level tubes are generally filled with pure alcohol for ordinary purposes; for trade purposes with methylic alcohol, which is much cheaper. For very delicate tubes sulphuric ether or chloroform is used. The sensitiveness of the bubble depends very greatly upon the mobility of the liquid with which it is filled, and to the quality of adhesion of the liquid to the glass. The relative mobility of the above-mentioned liquids is found by delicate tests with the bubble trier for small distances under the microscope at a temperature of 60° Fahr. Taking water as 100:—we find commercial methylic alcohol 22, absolute alcohol 13, sulphuric ether 5, chloroform 3,—that is, for equal small runs taken in 15 seconds of time. All bubbles appear to be more or less affected by temperature, particularly where the spirit is not nearly absolute. In the higher temperatures the bubbles are more active. The objection to chloroform, where it is likely to be subject to great changes of temperature, and where there is no provision made for regulating the size of the bubble—the means of doing which will be presently discussed—is that its expansion from heat is so great that it is very liable to burst its tube. It can therefore only be used with ordinary sealed tubes where these are small and strong. Sulphuric ether has the same fault, but in a lesser degree. 177.—The sealing of ground tubes requires the skill of a very experienced glass-blower, and is a technical matter on which no written instructions would be of value under any Fig. 48.—Colonel Strange's level tube. Larger image 178.—Colonel Strange's Level Tube.—These tubes, Fig. 48, are blown with an outward bead at each end of the tube, two outwardly screwed collars, F, being first placed over the tube before the blowing. The tube is then ground to curvature. A plug, S, is formed for each end of the tube from a plano-convex lens, ground to a bevel on the plano side, and also ground into the end of the tube as a stopper. A cap, C, is screwed over the end upon the collar. The springiness of the cap keeps the stopper always tight. As there is no blow-pipe used after the grinding, the tube remains constant as it is ground, or it can be adjusted by grinding to any desired sensitiveness. This cap, for security, is better covered with silk tied over it, and afterwards well varnished. In this class of tube there is always a little risk of evaporation. The system is not adapted to instruments to be used in the open air. 179.—Chambered Level Tubes.—As the run of a bubble varies slightly with its size, for exact purposes and Fig. 49.—Bubble with supplemental air-chamber. Larger image Fig. 50.—Colonel Scott's patent protected bubble. Larger image 180.—Extra Strong Level Tubes.—Colonel Scott's very ingenious device of enclosing a level tube within another tube of thoroughly annealed glass will be found valuable in all cases where the tube is much exposed, or where it is difficult Fig. 51.—Artificial horizon level. Larger image 181.—The Level Tube may form a Complete Instrument in itself.—In this case the lower surface is ground to a flat plane to rest on any plane surface. This level is generally contained in a small pocket case, and is most convenient for adjusting instruments to level. It is commonly used with the black glass artificial horizon, to be described. 182.—Mounting Tubes.—Level tubes when applied to instruments are generally mounted in brass covering tubes. Small level tubes under 2½ inches are conveniently mounted in such tubes with a fixing of slaked plaster of Paris inserted at each end of the brass tube. Larger level tubes should be bound round with thin paper pasted round the ends, which is allowed to get quite dry, to be afterwards fitted to the brass tube with a file. Fitted in this manner the tubes admit of adjustment to the difference of expansion of the metal and glass by change of temperature without distortion. There is no objection, however, to thoroughly fixing one end of the It is convenient in mounting level tubes to place white glazed paper under the bubble to reflect the light that passes through it to ensure better observation. 183.—In Fixing Undivided Level Tubes, or replacing them in instruments, it is important to observe that the side with the test mark, which is a small ground facet, should be placed on the top. 184.—In the Use of Level Tubes generally, it is not well to have them of greater sensitiveness than the general construction of the instrument upon which they are placed permits. Thus the centre of a surveyor's level that may be under constant strain from the unavoidable inequality of the pressure of parallel plate screws, will appear never to reverse properly if it has a very sensitive bubble, the cause of the irregularity being entirely due to the distortion from the strain on the vertical axis of the instrument. The same irregularity occurs in a lesser degree with the Y's of a theodolite, where these and the collars become corroded by exposure. The optician often gets an undue amount of credit for perfecting such instruments when he has merely replaced the sensitive bubble by a dull one—that is by doing what is really in this case the best for the instrument. 185.—When an instrument that depends entirely upon the level for its possible working is to be used abroad, an extra tube should be taken, as the level tube is very generally more exposed and is more delicate than any other part of the instrument. The tube may not only be accidentally fractured with a slight bow, but even the heat of the sun's rays will sometimes burst it. 186.—Reading the Bubble.—The exact position of the capillary concave surface of the spirit in the tube is liable to deceive the observer by the difference of refraction and reflection it gives, whether the light is towards the right or left hand. Fig. 52.—Pocket lens and mirror. Larger image 187.—Circular Levels have been made tentatively for a long period. They consist of a worked concave lens fitted into a brass cell with indiarubber seating, the glass being secured by burnishing over a bezel. This construction Fig. 53.—Hicks' patent circular level. Larger image 188.—Surveyors' Levels, of which there are many forms, consist essentially of a telescope with a diaphragm at the mutual foci of the objective and the eye-piece, the axis of the telescope being placed in a direction truly parallel with the crown of a sensitive level tube. The telescope with its level is mounted upon metal frame-work, carried up from a vertical axis upon which the telescope rests. The vertical axis is adjustable in relation to the axis of the telescope, so that they may be brought perfectly perpendicular, the one to the other. The whole instrument is also adjustable to a position of verticality of its central axis, and the horizontality of the telescope in relation to the surface of the earth in what is termed the setting-up of the instrument; so that when it is set up in this position levels may be taken from it in any horizontal direction from one point of observation, by rotation of the telescope about the vertical axis. Having these essential objects in view in the construction of the level, the form of the instrument may be varied as to details according to the mechanical skill and taste of the maker and the special demands of the civil engineer. 189.—In the design of a surveyor's level very important considerations are:—That the metal should be so distributed that every part is as light as possible, consistently with sufficient solidity to take a moderate amount of accidental rough usage, and ensure freedom from vibration; that the whole structure should be in equilibrium about its vertical axis when the telescope is extended at mean range, that is, at about the focus of three chains—this is a quality often neglected; that there should be sufficient light in the telescope, and that it should possess a firm and durable stand. Every form of level should embrace these qualities. 190.—The Oldest Form of Surveyor's Level is that termed the Y-level, so named from the telescope being supported in Y-formed bearings. This instrument was originally invented by Jonathan Sisson, a leading instrument maker of the 18th century. It was much improved and brought nearly to its present state of perfection by Ramsden, to whom practical opticians owe so much for many advancements of their science, and to his liberal publication thereof. This instrument is now very little used in Great Britain; but it still maintains its original position, to a certain extent, on the Continent and in America. In the eyes of the optician it is still the most perfect level, possessing all the instrumental refinements of adjustment he can desire. The reasons for its partial abandonment by the profession will be discussed further on. 191.—The Y-Level in a modern form is represented in the engraving below, Fig. 54. The Y's are shown at YY edgewise. They are supported by standards SR upon the limb L. The telescope is surrounded by two collars which are soldered upon it at positions exactly corresponding with the Y's. The collars are turned perfectly cylindrical and parallel on the surface with the axis of the telescope, and ground in a gauge-plate to exact size so that the telescope may be turned end for end in the Y's without altering the linear direction of its axis in reversing it. The telescope is held from shifting Fig. 54.—Surveyor's Y-level. Larger image 192.—The Y's are erected upon the limb, to which they are each fixed firmly by a clamping nut R at one end, and a milled head clamp at M. The telescope is held down by strap pieces, each of which has a joint at one end and a loose pin at the other, PP. The pin is secured from dropping when out of use by a piece of cord attached to a part of the instrument and to a loop through its head. At the top of the inner side of the strap-piece under YY a piece of cork is inserted in a cave. The cork by its elasticity keeps an equal but light pressure upon the collar of the telescope. It will be seen that by the above plan of holding the telescope, it is so far free that it may be revolved on its axis, by which perfect adjustment of the diaphragm to the axis of the instrument may be made in any direction. Fig. 55.—Section of parallel plate and vertical axis—arrangement of Y and other levels. Larger image 193.—Parallel Plates as a mode of adjustment of the vertical axis will be first described, as they present the oldest form of setting up adjustment. The vertical axis of the Y-level was formerly carried tapering downwards, and the upper parallel plate was placed at about the centre of the The diaphragm of the telescope of the Y-level is generally webbed with plain cross webs. The diaphragm and webs were described arts. 99 to 106. 194.—The Setting-up of the Y-Level is necessary to be understood before the instrument can be adjusted. The same description which answers for the setting up for adjustment will also answer for the setting-up of the instrument in the field for actual work. In this description it will be convenient, therefore, to consider the instrument as being in this case in adjustment as it leaves the hand of the maker. The after adjustments will be presently taken as from the original state of the instrument, as the maker has to do them in the first instance. Practically, the civil engineer has only to make slight differential adjustments at any time, as an instrument, by the solidity of its construction, will retain the general adjustment nearly, upon which further adjustments take more the nature of final corrections, which become necessary only from accidental causes. 195.—Setting-up of the Y or other Level with Parallel Plates.—The tripod stand is opened out so that the legs stand, if on level ground, inclined towards the centre of the instrument at an angle of about 70° to the horizon. The toes of the legs are each separately pressed into the ground sufficiently to make the instrument stand quite firmly. The instrument is then taken from its case and screwed down tightly upon the tripod head. 196.—The Eye-piece is Adjusted, art. 108, by sliding it gently in and out until the webs can be seen most distinctly. On a bright day a white pocket-handkerchief may with advantage be thrown singly over the object-glass to prevent any confusion from objects in the field of view during the focussing of the eye-piece. For the setting-up adjustment of the telescope, it is brought in position to lie directly over one pair of parallel plate screws, Fig. 55, PS, SP. Fig. 56.—Diagram plan of parallel plate screw milled heads. Larger image 197.—The milled heads only of these screws are represented in plan in the diagram Fig. 56, aa' being the opposite pair over which the telescope will be assumed to be at first placed. The level tube is now brought to adjustment by bringing the bubble to the centre of its run by means of the parallel plate screws aa', by taking the milled heads of these screws, one between the ball of the thumb and forefinger of each hand, and rolling them simultaneously the one in one direction and the other in the reverse. This action tips the axis of the telescope in one direction or the other. Thus by the screws being rolled inwards, as shown by the direction of the arrows in the diagram, the left-hand side of the instrument would be raised. If turned the reverse way, the right hand end would be raised. The opposite end, from that to which the 198.—When the level tube is adjusted over the screws aa' it is then placed over bb' and adjusted in a similar manner, returning again to the position aa' for final adjustment. When the level is in perfect adjustment the bubble should stand in the centre of its run in making a complete circuit of the horizon by rotation of the instrument upon its vertical axis. 199.—In and during the setting-up adjustment it is most important that the screws should not be made tight enough to cause, by their pressure upon the parallel plates, distortion of the vertical axis. Should this occur, the instrument will not level in all positions by the same setting. The action of the screws also, from the great elasticity of the metal, should distribute the pressure about equally between the opposite pairs aa' and bb'. The difficulty of accomplishing this with certainty makes another form of adjustment, with three screws only, preferable for setting-up, which will be considered further on. Where the instrument is set up for use, if the adjustment of the bubble be fairly correct to the centre of its run, the reading of the staff may be sighted and the telescope brought to true focus upon it by moving its milled head until the divisions of the staff are as sharp as possible, and then moving the eye upwards and downwards to be sure there is no error of parallax, art. 109. After this the final adjusting of the bubble should be made, noting particularly that there are the same number of divisions in its run on each side from the centre if it is a divided bubble. 200.—Adjustment of the Axis of the Telescope in true parallel direction with the periphery of its supporting collars in its Y's. This is performed entirely with the four capstan- 201.—It is sometimes inconvenient to adjust out of doors: this may be performed very well indoors. By daylight a small cross may be made with ink on a sheet of white writing-paper for the sighting object, which should be placed at as great a distance as convenient, say 20 or 30 feet. By night a pin-hole may be made through a piece of paper and a candle or a lamp be placed behind it. 202.—Adjustment of Vertical Axis.—For this the eye-piece is first brought to focus on the webs. The telescope is then placed directly over one pair of parallel plate screws opposite each other, and the instrument is levelled. The Y's are then opened out; and the telescope is directed so that the intersection 203.—Adjustment of the Level Tube.—The telescope is placed as before over an opposite pair of parallel plate screws, and these are adjusted until the bubble is in the centre of its run. The telescope is then turned half a revolution, so that it is placed over the same pair of screws in the reverse direction, and the displacement from the bubble from the centre is now noted. The capstan-headed bubble screws at the end of the level B are then adjusted to one-fourth of the difference observed, and the parallel plate screws are adjusted for the 204.—If the level is to be adjusted by night, this can be done very correctly by a fine cross drawn on paper placed on a wall, with a candle or gas burner shining brightly on it at twenty feet or so distance from the instrument. For this adjustment by night the instrument must be well constructed, as the tubes require drawing out to their full extent for focussing near objects. If the tubes are not quite straight, the object-glass suffers considerable displacement in the drawing out, or technically droops, which is a very common fault in badly-made instruments. 205.—Where webs are used for the reading, they are liable to become baggy or dirty, art. 101, and very frequently to break; nothing can, therefore, be more useful than to be able to re-web a stop in the evening, with command of the easy and certain means of readjustment described, when far from the optician's aid. 206.—As the Y-level is so perfect in its arrangement for adjustments, and so nearly meets the optician's ideal, it will be well to inquire what are the objections made to its use by the majority of British surveyors. The first and most important is that it possesses so many loose parts, to which the practical man honestly objects. The author was, many years ago, when Y-levels were more popular, trying to persuade a cautious practical surveyor who appeared to be very anxious for the certainty of his work, and who was going abroad, to take a Y-level instead of a dumpy one he was selecting, when he had his arguments stopped by the following question:— 207.—Improved Y-Level.—The above-described defects the author has tried to remedy by a modification of the Y arrangement, by forming the Y's with much broader bearings, and instead of the old loose pins screw fastenings are fitted, which firmly lock the telescope in position with the Fig. 57.—Improved Y-level. Larger image 208.—Perhaps, upon the whole, the conditions which formerly rendered the Y-level undoubtedly the best practical level have so much changed that the more solid construction of the dumpy may entirely supersede it, as it seems likely to do in modern practice, and the optician will lose his ideal. Some reasons for this may be stated, but whether sufficient is a question. The manufacture of object-glasses of good 209.—Further, with a well-centred object-glass, as it leaves the hands of the scientific optician, and a solidly constructed adjustment to collimation being provided for in the making of a level, true working may be done even if there is a small error in the collimation. The late William Gravatt, C.E., was of opinion that firm construction, compact form, and plenty of light in the telescope were more important than easy facilities of adjustment. There is no doubt he found the less open adjustments the better in the hands of the imperfectly trained assistants who were pressed into service during the railway mania of 1848. At any rate, at this period we have his invention of the "Gravatt," or, as it was afterwards termed, the "Dumpy" level, which has remained with us with slight modifications in its mechanical parts and with increasing popularity until the present time. The late Mr. Troughton, recognising the same facts, also made a level in which there was no adjustment to the supports of the telescope after Fig. 59.—Dumpy level. Larger image 210.—The Dumpy Level.—One of the most important structural improvements made by the late William Gravatt in his dumpy level, was the addition of a cross bubble, shown end-view in Fig. 59 at CB. This improvement over the old form of Y-level permitted the setting-up of the instrument to be completed approximately, without turning the level 211.—The Dumpy Level of modern form is represented in the engraving, Fig. 59. It consists of a telescope, fully described art. 94, which carries a ray shade RS at the object-glass end, to work in the field to eastward or westward facing a low sun. The eye-piece EP is adjustable to the webs in the telescope by pressure in or out. Two straps or bands are accurately fitted and soldered round the tube of the telescope; one of these carries a hinge joint, and the other a pair of locking nuts to support the level tube GG, and at the same time permit its adjustment. The level casing tube has two three-quarter bands, which slide upon it, pointed at one end GG: these adjust to the length of the bubble for changes by temperature. The lower part of each strap-piece is left a solid block of metal, to give very firm support to the telescope as it rests upon the limb L beneath. The limb may be either a Fig. 60.—Attachment of telescope block to limbs. Larger image 212.—It will be seen that by this means firm adjustment may be made either by raising or lowering one end of the telescope, as also by a lateral rocking motion should the web or bubble not be quite to position. This plan is certainly moderately solid, and little fault can be found with it, except that a little torsion may be put on the telescope by unequal screwing, and that it appears slovenly in leaving an open gap between the limb and block; therefore the author prefers in his own form of level, which will be presently described, that the block be solidly fitted down upon the limb, as is shown in the section Fig. 60, and the telescope be placed permanently exactly parallel with it. If the vertical axis be once fixed truly perpendicular to the axis of the telescope as solidly as possible there is very little risk of a bell-metal centre of ¾ inch or so diameter being bent; therefore all parts may be closely 213.—As the telescope of the dumpy level does not possess any simple means of determining the accuracy of the fitting of its sliding tube, it is a very important point in these levels that this fitting should be good, so that the object-glass does not droop when extended. For this reason the inner sliding tube of the telescope should be as long as possible, and its adjustment by the rack sufficient to bring an object in focus at 15 to 20 feet distance. This point is sometimes neglected. The author was once amused by a young surveyor bringing him an invention, which was to fix two points by the side of the telescope to enable him to read at short distances. It was seen on examination of his own level that his telescope, a badly-fitted one, would not read at half a chain, hence the ingenuity of his invention. In some cheaply made levels the solid ring fitting to the telescope, above described, which connects the limb firmly with the bubble tube, is replaced by blocks soldered on the telescope with soft solder: the method is very unsound from risk of imperfect soldering. The blocks are very liable to become loosened by a jar. 214.—The diaphragm of the dumpy level is generally webbed with two vertical webs and one horizontal. In use the image of the staff is brought between the vertical webs, which indicate whether it is held upright. The upper margin of the portion of the horizontal web between the two vertical ones is the index of level to which all readings are made, either for adjustment or for reading the levelling staff in the field. The somewhat loose and slovenly four-screw adjustment Fig. 61.—Diaphragm of dumpy level with webbed stop. Fig. 62.—Same, with stadia webs. Larger image 215.—Subtense or Stadia Webs.—It is very advisable in all levels to have two extra webs, or lines cut on glass, placed one on each side of the central horizontal web or line, fixed at such a distance apart that the image of 10 feet of the staff when placed at 10 chains distance may exactly cut the inner space between the lines. These webs or lines may be used as a means of measuring distances often more exactly than can be performed with the chain if the surface of the land is irregular; or, in any case, they form a good check upon chain measurement. If the webs or lines are separated so as to subtend an arc whose chord is 10 feet at 10 chains, it is easily seen that 1 foot of the staff will represent this chord at 1 chain, and that each ·01 of the foot on the staff will represent Fig. 63.—Tripod. Fig. 64.—Section of one turn-up leg of the same. Fig. 65.—Section of tripod. Larger image 216.—Tripods, or Stands.—This matter was deferred when describing the Y-level. The same form of tripod is used for both Y-level and dumpy. In this country the tripod is generally made of straight-grained, well-seasoned Honduras mahogany, which stands better than any other wood. When the tripod is folded up for carrying or for putting by it forms a cylindrical pole which is bellied out at about one-third its length from the top, and diminishes downwards and upwards from this point. For a 14-inch Y-level or dumpy the dimensions of the tripod are about 3½ inches at its greatest diameter when closed, tapering off to 2½ inches at both the top and the bottom ends. For a 12-inch level the section is somewhat less. Each leg of the tripod takes an equal section of the cylinder, the inner angle meeting in the axis being at an angle of 120°, as shown in section Fig. 65. Shovel-pieces are shown 217.—There is a little difference of opinion as to the form of the woodwork of the tripod for 14-inch levels, some preferring an open framed stand in place of the solid form shown in section Fig. 65. These open framed stands are not so compact to carry, and, as the author thinks, unnecessary for levels of 12 inches and under where the tripod head is solidly made. They are well adapted for larger levels and for theodolites, therefore the description of a framed tripod will be deferred to the discussion of these instruments further on. A few engineers prefer yellow pine for the tripods instead of mahogany: this is much lighter for its relative stiffness, but 218.—The lower points of the legs, technically toes, are pointed to an angle of about 60°, and are shod on the insides with steel plates to bite the surface upon which the tripod stands when the legs are extended for use. Two brass rings slip over and bind the legs together when the tripod is out of use. 219.—Many years ago the author introduced the plan of having one of the legs to turn up at about 1 foot distance from the toe. This is shown Fig. 63 at A, and in detail section Fig. 64. The joint is made perfectly firm by a winged screw at S, which screws from a boss cast on the hinge J to a solid metal shoe P. When the leg is turned up, the screw fixes it in the female screw S. This plan is very convenient for use in mountainous districts, as it enables the level to be set up fairly well without an uncomfortable angle to any of the legs, or risk of the instrument toppling over. This plan is now nearly superseded by a ball joint as a part of the setting-up adjustment. The tripod head shown under the level of Fig. 59 is by no means the best, but it is the easiest made therefore, it is the general trade form in use, both for the level and theodolite. Some very superior forms will be discussed further on in description of the instruments to which they are attached. 220.—The adjustments of the Dumpy Level.—As this instrument does not possess the means of revolving the telescope upon its axis as with the Y-level, the adjustments are somewhat more complicated, and are performed in an entirely different manner when they are to be made by the civil engineer. The differences are not so great in the hands of the optician, as he generally possesses a movable pair of Y's upon which he can adjust the telescope conveniently for collimation within his own works, by supporting the telescope The bubble is handier to work with when adjusted to reverse in the centre of its run, but it does not really matter, as equally accurate work can be done with it in any other position. Should the bubble not reverse in the centre of its run, adjust the instrument by the levelling screws until it reverses in some position. Say you start with bubble in the centre, and on reversing, it runs towards the eye end of the telescope six divisions, then alter the levelling screws until it is only half this, or three divisions towards the eye end, then, if properly levelled, the telescope will make an entire revolution with the bubble in that position, which will prove that the axis is vertical. The bubble can now be adjusted by the opposing nuts at the one end by means of the tommy pin (provided in the case) until it is in the centre of its run, and it will then reverse in that position instead of three divisions towards the eye end. 221.—Adjustment to Collimation.—Upon a fairly level piece of ground the staff plate, fully described further on, is trodden well down on the ground, and the level is set up at say 3 chains from this, in which position the staff is read as a back sight. Now in the opposite direction in the same line, at 3 chains distance from the level, a second staff plate, or in defect of this if the surface be not firm, a stake or a boulder, is driven firmly down in the earth, and the staff is placed upon this erect and face to the instrument as a foresight. The instrument is turned half round and the second station is read. These readings of the staves taken will be truly level with each other, if the axis of the instrument has been set up quite vertically, so that the bubble has kept its centre in all positions. Fig. 66.—Adjustment of dumpy level. Larger image 222.—In the dumpy level, as it leaves the hands of any respectable maker, the subsequent adjustments required can never be great, unless the level has suffered a serious fall so as to bend the limb. The rewebbing the stop, if carefully done, would require only a slight readjustment; but it may be convenient to give an exact method for extreme cases, which may be given in detail for clearness, and at the same time we may also consider the influence of the curvature of the earth. 223.—Original Adjustment of the Dumpy Level to Collimation with consideration of the Curvature of the Earth.—Suppose the readings of the two levelling staves at 10 chains apart, taken with the level placed at intermediate distance as before, read 7·50 and 4·50, and that we now place the level linearly at 1 chain outside the first reading and it reads the near staff 6·50 and the distant staff 5·50, by the inclination of the ground, this would be a + and a - reading; but we require both readings of one sign, and as the distant staff reading is much too high, it is clear we require - readings for correction. The correction will be of the difference of 7·50 - 6·50 = -1, 4·50 + 5·50 = +1, difference = 2. That is -2', as our readings are - and as the -2' is in 10 chains, at 1 chain the distance of - the near staff = -·2, and 11 chains the distant staff = -2·2. The correction will therefore be for the near staff 1 chain distant 6·50 - ·2 = 6·30, and for the distant staff at 11 chains 5·50 - 2·2 = 3·30 = -1·2 below each of the first readings. If the telescope be now collimated to the near staff reading 6·30, by adjusting the screws immediately under it for distance between the limb and the telescope, and the bubble be readjusted to the telescope without moving the instrument or touching the parallel plate screws, the adjustment will be perfect, less the small error due to the earth's curvature in 1 chain. If the telescope be adjusted to the distant staff 3·30, curvature of the earth will be corrected by the level for 11 chains, which is 0·0106 foot or ·01 nearly, the smallest reading we have on the staff. 224.—It was claimed by the late William Gravatt for his method of adjustment, 225.—Where space is not at command and curvature correction is not desired, adjustments of the level may be made with care at 1 chain distance on each side of the setting-up of the level with one staff only, which can be moved from one stake to the other, and with the final setting-up of the instrument at 1 chain distance from these stakes as before, Fig. 67.—Collimator for adjustments to horizontality of the telescope. Larger image 226.—Collimator.—Optical manufacturers in populous districts, and some observatories, as that of the India Store Department at Lambeth, adjust by means of the collimator by the exact method due to the late eminent German mathematician, Carl F. Gauss, which is hence termed the method of Gauss. The collimator consists of any good telescope permanently adjusted to solar focus, with a webbed diaphragm placed in the focus, where it may be illuminated by a lamp or by the reflection of daylight, and provided with means of bringing the telescope to a level position. As the collimator is generally constructed, it consists of an 18-inch telescope, Fig. 67, of the same description as that used for a Y-level, described art. 94, in which the telescope is surrounded by accurately turned collars formed to rest in Y's. The Y's are supported upon a heavy cast-iron stand, of somewhat triangular form, of nearly the length of the telescope, about 6 inches wide at one end and 2 inches at the other. The stand has two feet extended to the full width at the wider end, and one foot at the narrower end under the telescope. Each foot 227.—A lamp or gas flame is placed at a short distance from the eye-piece end of the telescope, so as to illuminate the webs that they may be distinctly seen when looking into the objective end of the telescope. In bright daylight, if there is a skylight over, a reflector will answer the same purpose. At the Lambeth Observatory a fine needle-point hole is used instead of webs. 228.—The instrument to be adjusted may be placed at any convenient distance from the collimator. For adjustment of a level, where the collimator is already in adjustment, the level is raised upon its stand until the axis of the telescope sensibly coincides with the axis of the collimator; then if the telescope of the level to be adjusted be focussed into the objective end of the collimator, the illuminated webs will be clearly seen; and if these webs be brought by adjustment of the level exactly to coincide with its own webs, the collimation lines of the two instruments are exactly parallel. In this adjustment it is only necessary to be sure that the vertical axis of the level is truly vertical, so that the bubble reverses without displacement, in which case the whole instrument must then be in perfect adjustment. 229.—It would be very difficult to use this method of adjustment if it were necessary that the axes of the level and collimator should exactly coincide. It is only necessary that they should nearly coincide, on account of the imperfection of object-glasses, which rarely work so well near the edge as towards the centre; otherwise any directly parallel position in front of the object-glass would answer, as the next diagram Fig. 68.—Diagram of collimation by two telescopes. Larger image 230.—When two levels are at command, one a Y-level, or even a dumpy in perfect adjustment, the one may be used as a collimator to the other by setting them up at a distance within their focal range on a firm basement floor. A candle or a lamp will give sufficient light to illuminate the webs of the instrument, which is used as a collimator, being certain, of course, that this instrument is first placed in level adjustment and set at solar focus, and that the instrument used as a collimator has a good object-glass. Fig. 69. Larger image Fig. 70.—Stanley's model 14-inch dumpy level. Fig. 70A.—Tripod. Larger image 231.—Improved Dumpy Level.—The writer has made some improvements in the dumpy level, which have so far met with very general approbation from the profession, Fig. 70. 232.—The further improvement, which the author considers of the greatest moment, is that the vertical axis is fixed directly and firmly upon the limb, and not through a loose screw fitting for separation at this point as in the ordinary dumpy. This is shown to be important in that, with the dumpy, where a loose screw is employed, any little difference 233.—Tribrach.—The setting-up adjustment of the instrument is upon tribrach limbs with three screws only. These screws can never strain the vertical axis, which in this instrument is somewhat deeper and more firmly made than that of the dumpy. In the old form of tribrach the points of the screws were held down by a spring plate placed above them. This plate, in carrying the instrument upon its stand over the shoulder, which is the most comfortable way if the stations are not far apart, was very liable to strain sufficiently for the screws to get loose. The author patented a much more solid method, by which the old spring plate is entirely dispensed with. In this plan each screw has a ball at the lower end, which is inserted in a tubular fitting formed in a solid tribrach, made of exact dimensions to take it. The tube is open on the upper side, as shown in longitudinal section H, Fig. 71. Many years' experience and the fact that numbers of makers have copied this form since the expiration of the patent, shows this plan to be perfectly successful. The general construction of the lower part of this level may be seen from inspection: L limb, fitted with compass; C axis, in one casting with the limb; S sprang, carrying the socket and supporting the instrument. PH shows the ball head arrangements to the screws. A central screw in this part detaches the tripod. One point is shown Fig. 71.—Details of Stanley's dumpy level: half elevation to left, half section to right. Larger image 234.—As the tribrach system of adjustment is of somewhat recent adoption to ordinary surveying instruments in this country, it strikes the stranger to it as being more difficult in use. It is really the most simple and expeditious system as is clearly explained by the foregoing diagrams, Figs. 72, 73 of the plan of a level, omitting its lower parts. Figs. 72 and 73.—Tribrach adjustment. Larger image 235.—The bubble of the level is placed parallel with two of the screws of the tribrach, that is as B and C, Fig. 72, and is adjusted to the centre of its run. It is then placed at right angles to the first position, so that the screw A comes directly under the bubble, to be adjusted by this screw only until it again comes in the centre of its run. Fig. 73 shows this second position with the screw A underneath. The level should after this read all round true, but it is well to try it round parallel with the different pairs of screws in all positions to give small adjustment if required. Where there is a cross bubble the level may remain for adjustment in its first position, but it is well to try it all round, as the long bubble is made uniformly the more sensitive. Fig. 74.—Ray shade. Larger image 236.—The Ray Shade to the telescope used in the above-described level has two narrow slits opposite each other at 180°. A zero line is carried from one slit to a line on the ray shade fitting when the slits are quite horizontal. Sights through the slits at zero enable an approximate cross-level to be taken. The edge of the tube of the ray shade is divided 30° on each side of the zero line to 2°, so as to take approximate lateral inclines of the surface of the land in levelling. This useful plan of cross-sighting was originally proposed by Gravatt. Fig. 75.—Stanley's platino-iridium point level stop. Larger image 237.—The most important variation from the telescope of the dumpy level described is in the diaphragm, where webs or lines of any kind are entirely done away with, and are replaced by a special form of index. This is represented in Fig. 75. The movable part carrying the opening of the diaphragm is Fig. 76. Fig. 77. Difference of reading with a web and a point, shown much magnified.. Larger image 238.—In early levels of improved construction, as shown Fig. 70, a difficulty was experienced in practice in bringing Fig. 78.—Stanley's improved dumpy. Larger image 239.—Stadia Points.—The author commonly makes the points, Fig. 75, VV' stadia points, by making the distance of the extreme ends of these subtend an angle, equal to 10 feet of the levelling staff at ten chains distance, or 1 foot of the Fig. 79.—Stanley's quick setting-up level. Larger image 240.—Quick setting-up Tribrach.—One objection has to be made to the tribrach over the four-screw system of adjustment, that the four-screw admits of greater inclination to the tripod, which is important in hilly countries. To remedy this defect the author designed a ball arrangement to the axis, which permits the level to be set 15° to the inclination of the tripod independently of the screw adjustment, so that the level, when the tripod is set at its best angle, may be brought immediately to nearly its final position. The arrangement is shown in the engraving Fig. 79. The axis carries a cup formed in the metal casting, which can be clamped down upon a ball-shaped recess formed upon the tribrach by means of a winged nut placed under it, the wings of which project between the tribrach screws. A very slight pressure is sufficient to firmly clamp the ball. This form of level is now very popular with civil engineers. With a point diaphragm and a tangent screw to the axis, not shown in the engraving, it is, in the author's opinion, the best practical level he has been able to design. Fig. 80.—Stanley's Engineer's level. Larger image Since the last edition was written the reviser of this work introduced, in conjunction with Mr. Stanley, his new solid bodied engineer's level, which has practically revolutionized the form of dumpy level and has proved such a success that more of this form are now made than all other forms put together. In this level, Fig. 80, the centre, body of telescope, object end and bubble fitting are all combined in one piece of gun-metal, so that although of vastly greater strength and rigidity it does not weigh as much as the old form of tubular body with its collar and stage. This does away with many separate pieces which are usually soldered and screwed together. It thus forms the strongest and most compact level yet made, and with ordinary care it will last in perfect adjustment a lifetime. The pinion for focussing is fitted to the side of the cast body, instead of to a tube, thus greatly increasing its firmness. Its form is equally adapted to the four-screw levelling if desired, as shown on next page, Fig. 81, in which it will be seen the four-screw levelling is of much improved form, giving greater strength and far more wearing and bearing surface to the levelling screws. Fig. 81.—Stanley's Engineer's level. Larger image The reviser has also patented a new form of spherical joint, which has met with equal favour. This improvement consists of a section of a ball (screwed to fit the stand head) Fig. 82.—Stanley's Engineer's level fitted with quick setting spherical lower plate. Larger image As ninety per cent. of the orders now for levels are for the form shown at Fig. 82, the reviser ventures to think that A further improvement has been made by making the diaphragms interchangeable, so that any form of diaphragm that is preferred may be instantly fitted without disturbing the adjustment, and when lines on glass are used it may be removed for cleaning, and replaced without interfering with the adjustment. The diaphragms illustrated below, Fig. 83, are usual forms, and it is recommended that when webs are preferred a glass one should be carried as a spare in case of accidents. E Stadia points for clamp and tangent levels. F Stadia points for ordinary levels. G Stadia glass diaphragm. H Webs. J Stadia webs. Fig. 83. Larger image 241.—The further discussion of the subject of high-class levels becomes somewhat difficult. Leaving out of consideration the levels sold by the trading optician, who deals in the commercial article but sometimes superadds a little fad, every genuine manufacturer has his pet plans of carrying out details, some of which may be very meritorious, but which could scarcely be described without a fuller discussion than our space permits. There is also, no doubt, a great number of mistakes that have been made in the construction of surveyor's levels. The direction in which the scientific optician generally fixes his attention is to give the advantages of the Y-level in the dumpy form, assuming the civil engineer holds a certain amount of prejudice against the use of the Y, for which, in its old form at least, the writer must admit that he Fig. 84.—Cushing's 12-inch improved level. Larger image 242.—Cushing's Level.—The level illustrated above, Fig. 84, by the late Mr. Thos. Cushing, F.R.A.S., Inspector of Scientific Instruments for India, would under any circumstances claim attention, from this gentleman's well-known high technical scientific attainments. It has also the merit of being in practical use in India at the present time. 243.—Cooke's Level.—An instrument somewhat equivalent to the above has been patented by Messrs. T. Cooke & Sons. In this, instead of the objective and eye-piece ends of the telescope only being reversible in the collar fittings, as in Mr. Cushing's level, the entire telescope reverses end for end in an extra outer tube, which is fitted between the collars. This tube also permits the rotation of the whole optical parts about the axis of the telescope for adjustment for collimation, although in a manner more frictional, and therefore more likely to disturb the instrument than in the simple Y adjustment. In this instrument, again, it is easily seen that it is the perfection of the Y-level, without its outward appearance, that is aimed at, and to gain this the weight is increased by extra fittings and double tubes, which are liable to become fixed by a slight dent upon the outer tube. Taken altogether it is not quite so convenient or so simple as the best constructed Y-level; but if it gives the adjustments the optician holds to be most important, in a disguised form it may be acceptable to the civil engineer. We may in this manner, perhaps, from the optician's point of view, count it a certain gain in the same direction as Mr. Cushing's level just described; but if we may accept the late Mr. Wm. Gravatt's ideas, already mentioned, the complication is unnecessary. 244.—A few other structural variations of details may be mentioned, as these are constantly cropping up as new inventions. The bubble tube is sometimes placed upon the stage instead of being upon the telescope. This is thought to protect it. It is not, however, so easy to read it in this position. The compass is sometimes made a loose part—when it is not required on the work its weight is saved. Various forms of locking screws are made to the supports of the telescope; these are only necessary to correct imperfect 245.—Supplementary Parts to Levels.—As a rule, supplementary parts fixed to the instrument, beyond the magnetic compass sometimes required, are very objectionable if the object of the level is to be levelling, as these additional parts inevitably increase the weight which has constantly to be borne in carrying the instrument. Supplementary parts have been carried, in various schemes, to the extent of combining the entire level with the theodolite, at the same time nearly combining the united weights of the two instruments. As a rule, professional men rarely care for complex combinations; and even after a limited popularity is granted to extra parts not absolutely required, these are generally finally abandoned. Mention of two such parts, therefore, only will be made, as these owe their introduction to the late William Gravatt, and are found applied to many levels in use, or at least contained in the case with the instrument. 246.—Bubble Reflector.—This was formerly placed upon all dumpy levels. It consists of a small mirror about 2 inches by 5/8 inch fixed in a frame that is jointed at its lower end to a short piece of tube partly cut away so as to form only a little over a semi-cylinder. This tubular part just clips firmly upon the brass casing tube of the spirit level. The reflector, when placed vertically on the level tube, can be adjusted by its joint, so that the run of the bubble may be observed by reflection in looking above the eye-piece to see Fig. 85.—Compact cheap form of dumpy level. Larger image 247.—Sight Vanes.—Two sight vanes are placed above the telescope, either as loose fittings or to hinge down upon the level tube. One vane has a vertical narrow slit and cross hair; the other has a window with a vertical horse-hair placed in its centre. This arrangement gives sight of distant landmarks in line with the direction of the telescope, upwards or downwards, beyond its field of view. A slider, fixed upon the window sight, reads at its upper edge into divisions cut on the vane, by means of which an approximate rate of forward inclination of the land may be taken. This sighting arrangement adds about half a pound weight to the instrument. It was useful with object-glasses of small field of view, but is useless with good modern glasses of wide angle. 248.—Lower-class Levels.—A level is often required by an architect or a contractor for works of limited area, where it is quite unnecessary to go to the expense of a civil engineer's level of refined manufacture. In such cases the level may only be used occasionally and under favourable circumstances, so that extreme solidity is not demanded, neither is distant view in the telescope required. The level generally made for such work is a simple dumpy, without cross bubble, compass, or any extra fittings, and with one eye-piece only. 249.—The instrument Fig. 85 illustrates the author's newest design for a simple level. It has a light form of tripod. The legs clamp directly between angle plates—these are not quite so portable or so neat as cylindrical legs, but they are easily made, very firm, and will bear considerable wear and keep in order. A still cheaper form is made with smaller telescope and turned legs for the tripod. Fig. 86.—Contractor's or builder's level. Larger image 250.—The illustration Fig. 86 represents the cheapest form of level with a tripod stand that has been constructed, Fig. 87.—Sighted reflecting pocket level. Larger image 251.—Sighted Pocket Level.—This consists of a tube, which is generally drawn of square section. A pin-hole sight is made in the closed end of the tube, Fig. 87, at E. The field end of the tube is left open. The sight is taken by looking through the centre of the pin-hole across the edge of the reflector R. A level with a small bubble is placed Fig. 88.—Pocket telescopic level. Larger image 252.—Pocket Telescopic Level.—In the above-described pocket level, where it is made short, the average middle-aged man will not have sufficient accommodation of vision to be able to see the bubble and the screen sharply defined simultaneously with the distant object to which the level is to be taken. In Captain Barrie's Fig. 89.—Reflecting level. Fig. 90.—The same construction in protecting case. Larger image 253.—Reflecting Level.—This simple level, Fig. 89, the invention of Colonel Burel, is one of the most portable. When it is used with a fair amount of care it will give good approximate results. It consists of a piece of parallel glass, which has half the surface silvered to form a reflector. It is suspended in such a manner that the glass hangs vertically by gravitation. The position of the mirror to the plain glass may be that shown in the engraving, or horizontally if preferred. The mirror, Fig. 89, is inserted in a solid metal frame suspended from a gimbal, which permits it to hang perfectly free to the action of gravitation. The centres of suspension are made with slightly-rounded knife-edges. A ring at the upper part of the instrument is placed over the thumb or finger to support the instrument when in use. A stout pin passes through a prolongation of the lower part of the frame, screwed or otherwise, which permits adjustment by filing to 254.—In using the Reflecting Level, it is held upon the thumb at about arm's length, and adjusted by raising or lowering the arm until the reflection of the pupil of the eye seen in the mirror is exactly bisected by the line cut by the mirror against the clear glass. The distant object seen in front, that cuts this sight line and the image of the pupil of the eye, will then be in true level position with the eye of the observer, provided the air is still, so that the mirror is not deflected from verticality. From the natural unsteadiness of the hand there is some little difficulty of getting this level quite free from oscillation. This may be obviated, or nearly so, by clutching a picket or staff with the hand and suspending the level from the thumb projected out for the purpose, or by resting the hand against a tree or other firm support. Capt. A. H. East, R.A., has suggested to the author a very capital device which he employs for hand instruments. This is to place the handle of a stick (or umbrella) in the waistcoat pocket, to clutch the body of the stick with the hand which holds the instrument, and to steady it with the other hand. In this manner the two arms and the stick form a tripod of surprising steadiness. 255.—Reflecting Level in Case.—In windy weather much greater exactness may be secured by placing the pendulous level, just described, in a tubular case, Fig. 90. The case is made of double tubes, so that the aperture cut on one side may by a half turn of the outer tube close and protect the instrument when out of use. The transparent side of the inner case is sometimes closed by thin glass tube of its own internal diameter. It is much better if made with two vertical sides glazed with parallel glass. When this form of instrument is used, it may be, if required, made to fit on the top of a light staff. The eye is then brought with much 256.—Water Levels.—The antique form of level, composed of two vials fixed on the ends of a tube and partly filled with water, by which a level is sighted in looking over the surface of the water, is still used to a limited extent in rural districts on the Continent; but the spirit level in some simple form is fast superseding it. The same principle of level, but with long tube, has been found convenient for the surveyor in measuring through close buildings, Fig. 91. Fig. 91.—Tubular water level with open vials. Fig. 92.—Browne's standard water level. Larger image 257.—Browne's Water Level Platelayers' levels and mechanics' levels generally are deferred to consider with useful hand tools and apparatus employed by surveyors in the final chapter. |
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