  INSTRUMENTS FOR MEASURING LAND AND CIVIL WORKS DIRECTLY—CHAINS—VARIOUS TELLERS—STANDARD CHAINS—ARROWS—DROP ARROW—VICE FOR ADJUSTING CHAIN—CAINK'S RULE FOR INCLINES—STEEL BANDS—WIRE LAND MEASURES—COMPENSATION SYSTEMS—LINEN TAPES—OFFSET RODS—PINE STANDARD RODS—RODS WITH IRON CORE—BEAM COMPASS RODS—COINCIDENT MEASUREMENTS—COMPENSATED RODS—BASE LINE APPARATUS—COAST SURVEY LINES—PERAMBULATOR—PEDOMETER—PASSOMETER—SOUNDING CHAINS—SOUNDING LINES—TELEMETERS—HAND RODS—RULES. 715.—The Instruments Generally Employed for Measuring Land are chains, steel bands, and tapes. Where roads are roughly measured, pedometers are commonly used. Where very exact measurements are required, rods have been used. Rough approximate measurements are obtained by stepping, with the use of the passometer to count the steps. 716.—Land Chains.—Although these are made in many qualities the forms vary very little. They are too well known to need much description. In the British Isles and some of our colonies the chain of 100 links, equal to 66 feet, the invention of Edmund Gunter about 1620, is generally used, 10 square chains (100,000 square links) giving the statute acre, presenting a decimal system of measurement much in advance of any other at the present time. The best land chains are made of steel, which is afterwards Fig. 330.—Land chain and arrows. Larger image All the best chains, whether of steel or iron, are made with long links formed by turning up the ends of a length of wire. Three small oval links are placed between each pair of long links. These three interval links are found to cause the chain to kink less than when only two are used. Each oval link is sawn through at the meeting line, which is brought up on one flat side of the oval in bending it from the wire. The saw-cut forms the point of adjustment. The small link is afterwards re-sawn and closed to shorten it, or forced open to lengthen it. There are generally four swivels in the length of the chain, two of which are at the handles: these prevent the chain from becoming twisted in turning the handles over in use. A swivel is shown Fig. 331 at S. Iron chains are sometimes galvanized to prevent rust. This process, however, makes the chain much more brittle, and cannot be recommended. It may be noted that all link chains lengthen with use. 717.—Tellers are small pieces of brass suspended to the Fig. 331.—Gunter's land chains. Larger image 718.—Inserted Tellers.—This form of teller is preferred by many, Fig. 332. It is much less liable to get dragged off, but it is not considered quite so distinct, and it is a little liable to get clogged with grass and weeds. Fig. 332.—Inserted tellers. Larger image 719.—The author's design for inserted tellers is shown Fig. 333.—Stanley's inserted tellers. Larger image 720.—Feet Chains are usually made 100 feet, more rarely 50 feet. They are generally made in foot lengths, but sometimes for flexibility are preferred in 6-inch lengths. They are commonly made of No. 8 B.W.G. steel or iron. The weight of 50 feet is 6 lbs.; 100 feet, 11 lbs. If made of light steel, No. 12 B.W.G., the 100 feet weighs 6 lbs. 721.—Mining Chains used in mineral districts are made generally 10 fathoms, or 60 feet, 6-inch links counted off by tellers in fathoms. They are made entirely of brass. The weight is about the same in brass as steel—No. 8 B.W.G., 9 lbs. Occasionally they are made extra strong, No. 7 B.W.G.; weight 12 lbs. In coal mines Gunter's chains are generally used. 722.—Metre Chains are made 20 or 25 metres long. They are marked with tellers at every two metres with a plain ring at the metre. The tellers are generally of the inserted kind, Fig. 332. In taking measurements the sign of the teller is doubled: thus the ordinary 1 or 10 is counted 2 metres; the 2, 4, and so on. 20-metre chains in light steel, No. 12 B.W.G., weigh 4½ lbs.; strong, in No. 8 B.W.G., 9 lbs. 25-metre, light, 6 lbs.; strong, 11 lbs. A land chain is generally secured for carrying by a leather strap with a buckle. Occasionally it is carried in a sailcloth bag with a strap over the shoulder. 723.—Standard Chains.—These are of the same form as the ordinary steel chain, but all the links are hard soldered after being adjusted link by link. They are not intended to be used for regular chaining, except it be for laying down rough base lines. Their special employment is to test chains, or to set out with two pegs on a straight piece of ground a standard length or station where the common chains in use may be tested daily. A standard chain is commonly enclosed in a box with a lock to prevent its accidental use for an ordinary chain. 724.—Arrows.—These are sometimes called pins. Ten form a set. They are shown with the chain in Fig. 330, and are commonly made of the same wire as the chain—No. 8 B.W.G. They are much better made one gauge stouter (equal to about 1/7 inch), and preferably of hardened steel than of iron. The common length is 15 inches. Where heath, stubble, or woodlands prevail 18-inch are better for use, and in some exceptional cases even 2-feet are very convenient. Surveyors going to new countries are recommended to take the longer arrows as well as those supplied with the chain. It is common either to tie a short length of scarlet webbing upon each ring of the arrow or to sew a piece of red flannel or bunting upon it to find it easily in long grass. Arrows are sometimes carried in a quiver with a strap over the shoulder, Fig. 334, which leaves the hands of the fore chainman free to remove obstructions where they occur. 725.—Drop Arrow, Fig. 335. Where ground is very hilly it is common to roughly level the chain by holding the lower position shoulder high, either by guess work or by using any kind of rough hand level or clinometer to ascertain this. The arrow is then dropped, and the point, held at first lightly in the ground, is pressed hard down or another arrow supplanted for it. The chain in this case is used in odd multiples of links as they occur, of which record is taken separately at each station. In going downhill a drop arrow answers very well. In going uphill a plummet to the last arrow is better. Some Fig. 334.—Quiver with arrows. Fig. 335.—Drop arrow. Larger image 726.—Examination and Adjustment of Chains.—Respectable makers send out chains tested to within half of one of the small links of standard, that is, within a quarter of an inch; but in use this error may increase either by the bending of the long links of the chain, when it becomes shorter, or in the more general case of friction from wear and from strain, by which it becomes longer. In London, standards are fixed upon the pavement in Trafalgar Square and at the Guildhall. These standards are also fixed at many municipal town halls. Surveyors very commonly lay down a standard on the pavement, or by pegs on a level gravel path. Where a peg is used it should be driven home nearly to the surface. It should if possible be made of a piece of heart of oak 12 inches long and about 2½ inches square. The standard length, which may be set off by a standard chain or new steel tape, should be from a saw-cut across the centre of one peg to a similar cut on the other. It is well also to have the centre space (50 links) indicated by a smaller peg. 727.—The Chain to be Adjusted should be first examined Fig. 336.—Stanley's vice for adjusting and repairing land chains. Larger image 728.—Chain Vice.—The links of steel chains can seldom be twisted open without breaking, and broken links cannot be restored by steel links. Iron links answer, but they are very stiff to twist open. Generally it will be found best for professional men to repair the chain with spare brass links. These wear very well. Where a smith is near with his vice and a light hammer the links are readily opened. It often occurs in open districts and abroad that no smith's shop is to be found. To meet these cases the author has constructed a special vice, as shown Fig. 336. This vice is let into a piece of hard wood—an old oak post answers admirably. In stone districts it is perhaps better to let it into a stone and fix it by pouring hot lead round it. The part B is used for an anvil for straightening the links. The vice V holds the link edgewise very firmly by bringing up the slide J by means of the screw S. The link may then be knocked open by the pane end of a light hammer. The link is closed again in the same manner. If the vice be 729.—Opening and Closing the Chain for Use.—The chain is most readily unfolded by taking the two handles in the hand and walking away from it as it lies on the ground. It is most convenient to place it about 45°, and half a chain length from the first station, each chainman taking a handle and moving to his position. The only danger in undoing a chain is from two chainmen taking one handle each and walking in opposite directions, in which case, if there happens to form a kink, the opposite movement of the two men will probably stretch or break the chain. In closing the chain it is taken by the middle links and folded up two links at a time till the handles are reached. If the links be placed consecutively in position round the axis formed by the first links, it may be folded up very compactly in a twisted form ready for the strap, by which it is carried, to be passed round it. 730.—Chaining is performed by two chainmen, termed the leader and follower. The follower, having pressed a stake into the ground for a starting point, then places the centre of the outside of the handle of the chain against it. The leader takes ten arrows in his right hand and one handle of the chain in his left, and walks directly towards a point which is to be the termination of the measurement, stopping at nearly the length of the chain, examining the chain to see that it is straight. He then places an arrow lightly outside the centre of his handle. The follower looks over this arrow to the distant station to see whether it is in direct line. If it be not so, he waves his right or left hand once, twice, or thrice for 1, 2, or 3 inches for movement to right or left. The follower picks up the arrows consecutively as left by the leader, and when he has the ten, 10 chains have been measured, which is then recorded in the field-book, or earlier Fig. 337.—Caink's rule for correcting inclines. Larger image 731.—Caink's Rule for correcting inclines in chaining is the invention of Mr. Thos. Caink, C.E., of Malvern, Fig. 337. It is made four-fold, each fold being one link. The link is divided decimally along the inside of the rule. On the outer edge of the rule there is a scale marked degrees, a part of which is subdivided where the scale is open to read closer, that is, to 20 or 30 minutes. These degree divisions, which read up to 16° on one side of the rule, indicate the space from the end of the rule to be allowed in addition for the same degrees of inclination of the land up to 4 links of measurement. On the opposite side of the rule the inclination scale is carried from 16° to 22° 10'. For these higher numbers the length of the rule is first set off, and then plus such part of the rule as is indicated by the position marked upon it of the required number of degrees. 732.—To Use Caink's Rule.—The follower has a clinometer of one of the kinds shown, Figs. 260 or 264. He 733.—In mountainous countries the eight links of the rule is insufficient allowance for common inclinations. Such countries are measured much more accurately by some system of subtense measurement, for which see Chapter XII.; but where a small piece of sudden steep inclination occurs half a chain may be taken, and the number of degrees indicated upon the rule be doubled, so that the full rule, instead of taking 22° only, will take 44°. 734.—Steel Bands for measuring, termed steel band chains, are made in various forms in this country, and sold by nearly all opticians. They are much lighter than chains of equal strength, and are made of standard length. They are also lighter to use, being smooth and without any projection. On the other hand the reading is less distinct than with the chain, and they need more careful usage in chaining. They also require oiling before being put by. From the thinness of the metal they are altogether more delicate and less durable than the chains for hard wear; but it is thought Figs. 338, 339, 340.—Steel bands and tapes. Larger image Fig. 341.—One link of steel band. Larger image 735.—The bands commonly used for land measuring are made 3/8, ½, 5/8, and ¾ inch wide, of Nos. 26 and 24 B.W.G. in thickness, respectively. The chain is divided into links by a small stud riveted through the centre of two small washers, a large stud being placed at the fives and an oval plate held by two rivets at the tens, which are numerically indicated in plain engraved figures, as shown in detail, Fig. 341 b, or perforated with holes indicating the number of tens. These band chains are made in links, feet, metres, or to any foreign 736.—Steel band measures are also made with divisions throughout, etched upon them with acid in such a manner that the divisions and figures stand in relief up to the original surface, whereas the new surface, which is etched back to form the ground, appears dull. The brightness of the figures and divisions on the dull ground makes them easily read. These bands are divided into links, feet and inches, metres and decimeters, or closer quantities either on one or both sides of the band as required. With the etched band there is perhaps a little risk of weak places from over-etching, although these bands are most carefully made, but perhaps this is not greater than in the inserted stud band, where weak places are necessarily caused by the loss of width at the points where the holes are made for the studs, wherein moisture hides after use in damp weather. 737.—The steel bands have handles the same as a land chain. They are wound upon a steel cross, Fig. 340. They are commonly placed in a wind-up case similar to that of an ordinary measuring tape, but in steel, provision being made that one of the pair of handles may be secured about the position of the axis of the tape for winding it up. In Fig. 338 the axis is made very large, so that the handle may be pressed in from an opening in one side of it. The newest idea is to cut a slit in one side of the plate up to the centre, as shown, Fig. 339. In this case the handle and band are put in from the side, so that the axis is no larger than is necessary to take the handle. A strap is placed on the side of the case for holding it. This is shown cut off to admit sight of the handle. 738.—The French make the handle generally T-shaped 739.—Wire Land Measures.—Where long open stretches of new country are to be measured, it is common to employ a steel wire chain, of 5 chains or of 500 feet in length, fitted with a pair of strong cross handles only. Fig. 342.—French land measure. Figs. 343, 344.—Marchant's 500-feet band. Larger image 740.—The author has made many chains of 500 links; in Fig. 344 a part of one is shown full size. This band, as we may term it, is wound upon a reel in an iron case, Fig. 343. A spring brake is placed at the position A, which holds the reel and prevents the band from springing out into loose hoops when it is run out. The 50 and 100 links are indicated by short lengths of brass tube placed over the band—single at the 50 links, but numerically indicated by number of bands as 2, 3, and 4 chains. In Fig. 344 a 50 and a 300 links are shown; weight, 3½ lbs. This flat, narrow, steel band chain was unknown until introduced to the notice of the profession in the first edition, 1890. It is now in very general use, and lengths may be had from Fig. 345.—Richmond's tension handle. Larger image 741.—Richmond's Tension Handle.—Various devices have been employed for giving equal tension to chains and bands to ensure equality of measurements. Salter's spring balance has been very commonly used attached to one handle of the chain to give a uniform pull, say of 15 lbs. This appears to answer very well. Mr. Richmond, surveyor, of Sydney, has devised a very simple plan for tension of light bands, which, being lighter and attached, is much more convenient than Salter's balance. This is shown Fig. 345. The band passes through a fitting in the centre of the handle, and a spiral spring is fixed to this and the band at a short distance along it. By pulling the handle a given tension can be applied, which is shown by the mark it reaches towards the end of the band. This is adjusted to standard length, and a small notch is placed in the centre of the end, from which a plummet may be suspended if necessary. The engraving is of a slightly modified form by the author, in which a thin tubular cap covers the free end of the band to save this exposed part from accidents. Fig. 346.—Copper case thermometer for suspending to a band chain. Larger image 742.—Chain and Band Thermometer.—Where very Fig. 347.—Littlejohn's temperature handle. Larger image 743.—The coefficient of expansion for steel between 32° and 212° Fahr. is about ·000012, which is less than ·01 inch per degree per chain. Temperature corrections can therefore be recognised only upon very exact work, appreciable only when long bands of the Marchant type, lately described, of from 5 chains to 10 chains in length are used. 744.—Mr. Littlejohn has patented an adjustable handle for temperature. This is divided for allowance for the 100-feet or other band for every degree Fahr. or centigrade. Fig. 347. The handle is set to the temperature as it changes during the day. It offers, perhaps, the highest refinement in ordinary land measurements. Fig. 348.—Stanley's repairing sleeve. Larger image 745.—Repairing Sleeve for Steel Bands.—The reviser has patented a sleeve which will be found useful, as by its use a broken band can be immediately and permanently repaired in the field without the use of tools. They are made to fit all sized bands, but it is necessary that the correct sized sleeve should be used. One of these sleeves is shown attached to a band at Fig. 348. In order to effect a repair it is merely necessary to clean the broken ends of the band, and insert them into the sleeve, then hold a lighted match under it until the soldering material is melted, when the repair is completed. The central hole in the sleeve is to enable the user to see when the broken ends are in contact, and the other two are to indicate when the soldering material is melted, which takes place when it either bubbles up in or runs away from these holes. Fig. 349.—Linen Tape. Fig. 350.—Small steel pocket tape. Larger image 746.—Linen Tapes.—This most useful implement, Fig. 349, is one of the most unsatisfactory measures the trader Tapes are divided into links, feet and inches, metres, and all measures as required. A decimal yard is commonly 747.—Steel Tapes.—Thin steel tapes, 3/8, ½, and 5/8 inch wide are in very extensive use. They are more accurate and more costly than linen tapes, but less flexible and less durable. Where dimensions are important they should always be used for short measurements. In all cases it is advisable for a surveyor to keep a steel tape for examination of the lengths of linen tapes in use. They are made to all the measurements of linen tapes. 748.—Pocket Steel Tapes 6 feet to 12 feet, Fig. 350, are used more generally by mechanical engineers. These tapes, which are very light, are held open by a catch, and closed by a spring. Fig. 351.—Jointed offset rod, top and centre. Larger image 749.—Offset Rods are generally made 10 links long, either in one piece or jointed in the centre with a bayonet joint. They are about 1-1/8 inches in diameter, diminishing towards the top to 7/8 inch, and made either of yellow pine or ash. A hook is commonly put at the top, Fig. 351, which takes the handle of a chain to draw it through a hedge or other obstruction. The author's plan of making this is shown at H. The lower end of the offset is shod with a steel or wrought iron socket point, so that it may be set up in the ground and used if required as a picket. Bands are painted alternately black and white at every link. Square or flat rods are occasionally used for the same purpose, but they are not generally so convenient. The offset is Used in the manner of an ordinary rule to take rectangular short measurements from the chain as it lies upon the ground, commonly in order to obtain the contour of irregular outlines. 750.—Measurement by Rods has become less general than formerly, from the greater accuracy of Konstat or Invar steel tapes, by which practically correct base lines may be laid down. For geodetic works requiring the greatest accuracy the bases have been laid with rods of various forms. These rods will be briefly described. It is only in the construction of iron bridges, roofs, etc., that rods are at present generally employed in the work of the civil engineer. 751.—Pine Standard Rods, made of straight-grained pinewood seasoned five or six years and then well soaked in linseed oil, make good standard rods. The ordinary length in use is 10 feet by 1¾ inches square. If the rod be used for butt measurement the ends are tipped with gun-metal in which a turned steel stud is hard-soldered. The stud is afterwards ground to true face in a lathe, and left of standard length at 60° Fahrenheit (15·5 centigrade), Fig. 352. A disc of brass 1 inch diameter is inlaid at every foot for 5 feet from one end of the rod, with a line at the true foot. These rods, after the work upon them is finished, are lightly French polished to keep them clean and to prevent the effects of moisture. The effect of temperature upon deal was found by Roy to be about the same as upon glass—·0000085, average of total length per degree centigrade, which is about three-fourths that of iron. Fig. 352.—One end of a pinewood butt rod. Fig. 353.—S—Block square. Larger image 752.—Where butt rods are used for continuous measurement, it is necessary that they be brought very carefully 753.—Angle-piece.—A solid angle-piece with two planes at right angles is very convenient for use with butt rods to give means of scribing the true length down to a surface, Fig. 353 S. 754.—Butt Rods with Iron Core.—Where rods are to be used for preparing iron work it is better to have an iron core through the rod, that may expand and contract with the metal on which they are used. The rods that the author has designed for this purpose are made out of a length of seasoned pine 2¼ inches square, sawn down and turned cut sides outwards to prevent warping. A 10-feet length of iron steam tube about ½ inch diameter is painted several times and then bound round with paper soaked in paraffin. This is placed in a pair of meeting grooves, as shown in section Fig. 355. The two pine flitches are cross-tongued together and glued up with the inserted tube between them. The tube has a turned steel cap placed over each end, Fig. 354, and this is ground in a lathe to true standard at the temperature of 60° Fahr. A steel pin is placed through the centre of the rod to indicate 5 feet. The finished size of the rods is 2 inches square. The author has made these rods in sets, consisting of two 10-feet and one 5-feet packed together with an angle-piece, Fig. 353 S, in a deal case. 755.—The 5-feet Rule is of steel, ¾ inch by ¼ inch, inlaid Figs. 354, 355, 356.—Butt rods with iron core. Larger image Where a long length is laid down for a base line or other purpose, it is better to take the thermometer reading at each measurement and defer correction to the completion of the work; the temperature errors may then be added together as a total, and the space allowance may become a measurable quantity. For example, say ten 10-feet lengths give by these united centigrade degrees, plus and minus, shown at separate readings + 167°, and that the standard of the rods is true at 15·5°. Then 167 - (10 × 15·5) = + 12° per foot total allowance, that is, 12° × 10 feet × ·000019 = ·0228 feet or ·2736 inches to be added. In measuring iron of course no correction has to be made. 756.—Beam Compass Measurements are occasionally preferred for iron work. In this case the beam is moved from centre punch mark to mark along a surface by the beam producing a scratch for the forward position in which to place Fig. 357.—Beam compasses. Fig. 358.—Standard scale. Larger image Fig. 359.—Coincidence rods. Larger image 757.—The Method of Coincidence in measurements by rods has often been applied to measurement of base lines. The plan consists in allowing one rod, or a lighter continuous part of it, to pass the other rod, so that a line cut to standard on one rod may be read into one on the other. The best plan to do this is to have a scale fixed along the face of one rod near its end, as shown Fig. 359, and to have an extension from the other end of the second rod to pass alongside this scale, so that two lines may be brought into coincidence. The rod B has a fixed scale b placed on top of it at one end. The rod A has a scale protruding from it. Fig. 360.—Bessel compensated rod. Larger image 758.—Compensated Rods.—The plan used by Bessel for the measurement of a base upon the shores of the Baltic in 1836 is looked upon as a model of the most perfect work of its kind. The rods were composed of two bars of iron having surfaces accurately planed, with a similar bar of zinc placed between them. The bars were laid one on the other, but not in contact, the surfaces being kept apart by glass plates, upon which they could slide with little friction. The linear expansion of zinc per degree centigrade is about ·0000292 (Fizeau); that of iron much less than half this—about ·0000119 (Thomson). The bars are attached to each other in such a way that the expansion of the zinc may act in the opposite direction to the expansion of the iron. The form followed for the construction is shown in Fig. 360 where II' are the iron bars, Z zinc. The length of the zinc required for compensation between the junctions is found in the equation— (S + Z)(0000119) - Z(0000292) = 0, S being the total length of the standard rod in feet, and Z the length of zinc in feet required for compensation. This plan is that adopted for the compensation of pendulums. For the 759.—Colby Compensated Rods, the invention of Major-General T. F. Colby, who was for twenty-seven years superintendent of the Ordnance Survey, upon which these rods were used. Each rod is composed of one rod of iron and one of brass, which are so arranged in pairs that the difference of expansion of these metals shall act to diminish the amount of entire expansion at the points measured, a quantity equal to its increase by temperature, in a manner to be described. The Rods are each made 10 feet 1·5 inches long, 5 inches broad, and 1·5 inches deep. Fig. 362 i is a side elevation of one rod, Fig. 363 ib plan of iron and brass rods, Fig. 365 ib perspective view. By this it will be seen that the rods are placed edgewise. The distance apart is 1·125 inches. They are supported in the middle upon rollers, Fig. 362 F. They are firmly fixed together at their centres by transverse steel cylinders, Fig. 363 RR' 1·5 inches diameter, each rod being left free to expand or contract from the neutral central point independently of the other. The neutral point is formed of a T-piece E, Fig. 363, fixed firmly on the bottom of the box bx. At the extremity, and at right angles to each of these bars, is a flat steel tongue, Figs. 364, 365 A, 6·2 inches long, 1·1 inches broad, and 0·25 inch thick, which projects 3·25 inches from the side of the iron bar i. The tongue A is jointed by double conical pivots at f and f', which form axes perpendicular to Colby Compensated Measuring Rods. Fig. 361.—End of rod mounted with microscopes, trestles and ground plate. Larger image The bars are placed in strong wooden boxes, to the bottoms of which are fixed the plates that hold the brass rollers upon which the bars are supported, Fig. 362 F, and the central stay E mentioned before prevents any displacement Colby Compensated Measuring Rods. Fig. 362.—Side elevation of point of support of rod. Fig. 363.—Side elevation of centre, with section of box bx. Fig. 364.—Plan of rods and compensating arm. Fig. 365.—Perspective view of the same. Larger image Two Rigid Tripod Stands Fig. 361 S are used to each of the rods placed under the rollers Fig. 362 F upon which the bars are supported in the box. The tripods carry a Compensated Microscopes.—The compound microscopes, Fig. 361 MM', used with the Colby apparatus form a complete separate instrument, consisting of two microscopes placed parallel to each other and united together for reading the rods when they are brought with their standard points the distance apart that separates the axes of the two microscopes. In the intermediate space between the two microscopes, and parallel with them, a telescope T is fixed on the same piece of apparatus, with adjustment for reading a point on the ground G perpendicular to the measuring rod. The microscopes are held apart by two bars of brass and iron 7 inches long, 0·5 inch broad, and 0·375 inch thick, which are placed at 2·5 inches apart and secured with the telescope, which forms the fixed centre, by collars to the bodies of the microscopes. The difference of expansion of the iron and brass maintains the separation of the microscopes at their foci at one distance with every change of temperature of the air. The object-glasses are of 2 inches focus. The microscopes are brought to adjustment and bearing by levelling on a tribrach whose base is fixed firmly to one of the rod cases, and by lateral adjusting screws. Special microscopes are used with each of the six rods of the Colby apparatus, and are distinguished by the letters M N O P Q R S. The weight of each compound microscope is 5 lbs. Very full particulars of the Colby apparatus with engravings of all parts, are given in "The Ordnance Survey Account of the Measurement of the Lough Foyle Base." In measuring a base line a piece of nearly level land is selected, and the rods are supported upon the trestles or The Colby system of measurement of base lines varied in detail has been employed by nearly all the nations of Europe and in America. 760.—Modern Base-line Apparatus.—The introduction of "Konstat" steel (highest grade Invar) tapes and wires has revolutionised the method of measuring base lines. These tapes offer a means which is far superior to anything obtained by measuring bars, because they combine the advantages of great length and simplicity of working, with more precision than the shorter laboratory standards, providing that suitable apparatus is used in applying them to their work. Base lines may now be rapidly measured with long "Konstat" steel tapes so that much longer lines are laid down than was formerly the practice when measured with bars, with the result that any errors that may be introduced do not affect the ultimate expansion so much owing to the greater length of the base. The coefficient of expansion of "Konstat" steel is under ·0000005 per degree Fahrenheit, so that provided accurate means of suspending the tape and reading it and transferring the readings to a plate properly let in the ground are used, we have a most exact and rapid method for this important work. The tapes are usually 100 feet or 30 metres long, but 300 feet or 100 metres are often used. The tapes are a few feet longer than these measurements, so that the rings are well clear of the reading lines. A silk cord is attached to these rings and passes over the end suspension supports, one of which is Fig. 366.—One of the two end supports of the band, showing tension weight, with cord running over the ball-bearing pulley. Larger image To prevent catenery light intermediate stands, as shown at B Fig. 367, are used at about every ten feet; these have a rising cross piece with guides which are adjustable for height and sideways to support the tape in perfect alignment. Having the tape properly suspended the reading instruments, C Fig. 367, are placed in position at either end. These are mounted on rigid-framed stands and provided with levelling screws, cross levels, transverse screw motions and movement in azimuth, with clamp and tangent motions and aligning telescopes. A powerful microscope is rigidly fixed over a little table over which the tape passes and reads its division with great exactness, coincidence with the division being made by the traversing screw. By the side of the reading microscope, and in exact collimation with it, a plumbing telescope is rigidly fixed, and this sights down to a transferring apparatus, D Fig. 367, which is over the plate let into the ground. Fig. 367.—Two reading and plumbing instruments, C, C; transferring instrument, D; and one of the adjustable intermediate supports, B. Larger image The transferring apparatus is a spring centre punch rigidly mounted truly vertical on a supporting plate having transverse motions, cross levels and levelling screws. The top of the centre punch has a small platinum disc let in a recess, and upon this disc very fine cross lines are marked. This apparatus is placed on the ground over whatever has been let in to receive the mark, it is then levelled and the cross lines upon the punch top brought by means of the transverse motion screws exactly to coincide with the spider web of the plumbing telescope, and in this position the centre punch is lightly struck with a mallet which marks the plate let in the ground in the exact position of the centre of the cross lines at its top, so that if now the transferring apparatus be removed the cross webs of the plumbing telescope would cut the dot marked in the plate by the centre punch. This method is far more exact than any hanging plumb-bob, as even if they are screened to prevent swinging very few hang with the point perfectly true. In laying down a base line No. 1 reading and plumbing instrument is set up and levelled over the starting A few 1-100th of an inch divisions, or 1-10ths of a millimetre, are divided on either side of one end division of the Konstat tape so that any allowance for expansion or contraction may be made under the microscope at the time, but with Konstat tapes this is very small indeed. With fairly level ground any slight differences of level can be allowed for in setting up the stands, so that the tape remains level; if the difference is too great for this the difference of hypo and base must be calculated. Thermometers are used, generally one suspended on the tape at each end. 761.—Perambulator.—A very ancient instrument, described by Vitruvius as being among the effects of the Emperor Commodus; it was used by hand, or attached to a carriage to measure distances. The instrument is at present used as formerly for measuring roads. Upon pavements and asphalt roads it measures accurately, where by reason of traffic it is sometimes a difficult or very slow process to use the chain. The plan of manufacture is varied considerably. The author makes the felloe of the wheel in segments of well-seasoned mahogany in two rings, Fig. 368. These are rivetted Fig. 368.—Perambulator. Fig. 369.—Details of registering box. Fig. 370.—Section. Larger image The Registering Part of the Instrument, Figs. 369, 370. The axle is protruded through the fork on the left-hand side, and thence through the registering box supporting one of its ends. The other end of the box is supported by a stud which fits into the side of the fork. The axle in the part contained within the box is cut into a screw, Fig. 370 S, of about sixteen threads to the inch. The screw works in the edges of a pair of discs R, placed one upon the other upon the same axis; these are cut on their edges with teeth to form worm wheels in which the screw upon the axis of the wheel The Measuring Box is covered with glass for protection. The box can be taken off by removal of the milled-headed screw at any time to set it back to zero, but in practice it is often found more convenient to spin the wheel round to zero or an even mile of the outer circle, and record differences of reading, if this can be done in the distance within the record of 13 miles of the lower disc. The screw and axis, which are of hard steel, should be occasionally oiled with watch oil to keep the perambulator in good working order. 762.—The reviser has designed a light form of perambulator on the bicycle wheel principle. It is shown at Fig. 371, and is very light and portable. The rim of the wheel is of gun-metal and is usually made two yards in circumference. It is fitted with a counter which denotes two yards to every revolution, and the distance is given in number of yards only. Fig. 371. Larger image 763.—Pedometer.—Used for roughly ascertaining distances passed over in walking. This ingenious instrument was the invention of William Payne in 1831 (patent No. 6078). It is the size of an ordinary watch, and has a similar face; but between the figures, which indicate miles 1 to 12, there Fig. 372.—Construction of pedometer. Fig. 373.—Face of passometer. Larger image 764.—The Registering Apparatus consists of a pendulum, Fig. 372, P placed horizontally by being supported by a delicate spring L to its highest position, where it rests against a stud. The action of the pendulum is caused by its following the motion of the body in stepping, until stopped by the foot reaching the ground, when the momentum attained by the pendulum carries it from its upper position of rest where it is sprung against the stop to its lower free position, where it is stopped by a screwed adjustable stud S, shown under it. The axis of the pendulum is free upon the axis of the ratchet wheel R. When the pendulum falls, a fine spring, fixed to its upper surface, drops its end into the teeth of the ratchet, moving over two or three teeth, which are held against retrograde motion by the spring pawl D. When the pendulum rises, the ratchet is moved forward the number of teeth that the spring at first slipped over. The ratchet is connected with a pair of geared wheels, not shown, the axis of the second of which forms the axis of the hand. In this manner each oscillation of the pendulum is communicated to the index hand. The ratchet is made with extremely fine teeth, so that by adjustment of the screw stud S a greater or less number of these teeth may be taken by one beat of the pendulum, and thus the mileage rate may be adjusted approximately to the step. This is done, however, very imperfectly, as the variation of the average steps of men amounts to one or two inches, and the difference from the number of teeth taken will scarcely indicate less than three inches in the step. 765.—Passometer.—This instrument was originally invented by the author as an improvement upon the pedometer (1868). The instrument, Fig. 373, is not intended to 766.—The average step may be estimated perhaps within 1 or 2 per cent. by training in walking several miles steadily, counting the steps, always remembering that we take shorter steps uphill and when we are tired. But the mean step of the individual under all the different circumstances is the only rule that can be followed. Fig. 374.—Sounding chain. Larger image 767.—Sounding Chains used for coast surveys are generally made of iron, but sometimes of brass. They are usually made of 10 fathoms entire length. The links are 1 inch, and the feet are indicated by tellers. The form of teller designed by the author is shown in Fig. 374 for the 3. A leaden weight, similar to that shown Fig. 375, is used upon the end of the chain—of 28 lbs., for ordinary coast work, or Fig. 375.—Sounding line and weight. Larger image A very elaborate apparatus with steel wire line has been made for deep-sea sounding by Lord Kelvin and others; but this subject is beyond the province of the present work. 768.—Sounding Lines, used for survey of shallow coasts and harbours, are made of water-laid line of fine green hemp, about ¾ inch circumference, Fig. 375. White tapes are inserted as tellers at every foot, and red tapes at every fathom. 3 to 6 fathoms are the ordinary lengths employed. If the water is shallow the fathoms are easily counted, but if thought necessary knots may be tied to indicate the number of fathoms on the red tellers. The weight is about 7 lbs. for 50 feet line, about 15 lbs. for 100 feet. The under side of the weight is commonly recessed to take tallow when it is desired to bring up a specimen of the bottom, if this is loose sand or mud. 769.—Coast Survey Lines.—For surveying distances, from point to point of soundings along a coast, lines of fine copper wire rope marked with tellers at 50 and 100 feet are commonly used. The line is generally allowed to rest on the bottom of shallow water, and is floated up by means of 770.—Telemeters.—These scarcely enter within the practical limits of surveying instruments, but as several attempts have been made to introduce their use it is necessary to mention them. The general attempt has been to measure a great distance, 1000 feet or more, by means of the angles subtended from the ends of a short base to a distant point. This base in the telemeter of Piazzi Smyth is 60 inches; Colonel Clarke, 72 inches; Otto Struve, 73·5 inches; and Adie, 36 inches. The angles are usually taken upon the principle of the sextant by coincidence of image. Very much greater success has been attained recently by Messrs. Barr and Stroud by means of their range-finder of 54 inches base. The author, as far as his information reaches, is assured that no instrument of the class is satisfactory for surveying purposes. Further, the subject is one to which he has devoted some study, and designed two telemeters. There are some instruments, as Colonel Gautier's telemeter used in the French army, which depend upon combined reflectors placed normally at 15° to 45°, as in the apomecometer, art. 693, but with a tangent screw to give a 771.—The simplest and best telemeter for surveying purposes is the subtense telescope, and all good, up-to-date surveying instruments have their telescopes so fitted, but for those who do not carry an instrument with a telescope the reviser has designed a small subtense telemeter, Fig. 376, which consists of a small telescope fitted with subtense points, and mounted in a collar which has vertical and horizontal motions and a centre socket to fit a Jacob's staff. The stadia is set to read 1 in 100. The telescope has rack and pinion focussing and may be revolved in its socket so that the stadia rod may be read held either horizontally or vertically. It is packed in a leather holster case, and a four-fold 10-feet spring-pointed stadia rod is supplied with it divided into feet, tenths, and hundredths. Fig. 376. Larger image 772.—Hand Rods, although used more generally by building surveyors, are extremely useful also to the civil engineer and land surveyor for town work among buildings and in mines. They are made 5 feet in length, less generally 10 feet. The 5-feet are made of single blades of lancewood or of two jointed to fold. The 10-feet are always jointed and made much stouter than the 5-feet. The 5-feet are generally sold in pairs. Fig. 377.—Ordinary 5-feet jointed rods—plan and section of joint. Larger image 773.—Ordinary 5-feet Rods are divided to every 3 inches, with feet only stamped with numbers, as shown Fig. 377. Where the rod is jointed the best form of folding joint is shown in the figure in section and plan. The spring S is sunk into the face of the rod at the joint on one side, and springs into a groove (housing) in the other side so as to lock the joint when it is either open or closed. The most useful dimension for the rod is 1 inch by 1/6 inch. Rods are nearly always made of lancewood, but they are preferred dyed black for neatness by many surveyors. A pair of rods is usually carried in a cowhide case. They are also often carried in the stem of a walking-stick hollowed out for the purpose. The rod or rods in this case are made much lighter, generally ½ inch by 1/8 inch for a pair of rods, or 7/16 inch by 7/32 inch for a single rod. The single rod is to be preferred in this case for its extra strength. 774.—Fully Divided Rods.—The author has made rods for many years divided to single inches. These measure from both ends—one end direct as Fig. 378 and the other end reversed by turning the rod over as Fig. 379. By this plan Figs. 378, 379.—Stanley's surveyors' rods. Larger image 775.—Connecting Link for Rods, which weighs only 1 oz. and may be carried loose in the pocket, is often found convenient for measuring heights, as it permits the ends of a pair of rods to be brought together, Fig. 380. By this means the arm will raise the rods about 7 feet, and with 10 feet, the height of the pair of 5-feet rods, this will make 17 feet of measurement. When the 10-feet is set against a wall, its height, if 20 or 30 feet, may be guessed very approximately by standing at a distance from it. Fig. 380.—Connecting link for rods. Larger image Fig. 381.—Slip jointed rod. Larger image 776.—Slip Jointed Rod.—This form is less general, but it is a very convenient form of rod. The jointing is effected by two loops which are fixed to the centre end of one part of the rod in such a manner that the other part may slide through the loops. When the rod is extended to 5 feet there is a stop which prevents further extension, and a spring to keep it at this exact position, Fig. 381. The outside of the rod is 777.—Brace-piece.—A 10-feet rod is sometimes made with a brace-piece, which folds up inside the rod. This brace-piece is jointed to fix both half rods to 90° when it is desirable to use the rod as a square. Fig. 382.—Civil engineer's rule. Larger image 778.—Civil Engineer's Rule is made fourfold in both boxwood and ivory, Fig. 382. The most convenient size is 1 inch wide. Some of the profession prefer them narrow for lightness—¾ inch; and some wide for strength—1¼ inches. This rule is generally well made, with German-silver joints and outside joint-plates. The divisions placed on the rule outside are inches in eighths and tenths; the inside, the ordinary architects' scales, 1/8, ¼, ½, 1, and four chain scales, 20, 30, 40, and 50. A 10 is got by halving the 20; 60, by doubling the 30. A protractor reading to 5° is divided on the head. With silver joints and in fine ivory this rule is often made a presentation instrument. |
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