  THE MAGNETIC COMPASS AS A PART OF A SURVEYING INSTRUMENT OR SEPARATELY—BROAD AND EDGE-BAR NEEDLES—MANUFACTURE OF THE NEEDLE—MAGNETISATION—SUSPENSION—DIP AND ADJUSTMENT—LIFTING—INCLINATION—DECLINATION—VARIATION—CORRECTION—COMPASS-BOXES—DESCRIPTION OF COMPASSES—RING COMPASSES—TROUGH COMPASSES—PRISMATIC COMPASSES—STAND—SURVEYING WITH COMPASS—POCKET COMPASSES. 116.—The Magnetic Needle, which forms part of a great many surveying instruments, is made of the form adapted to the special purposes of the instrument in which it is placed. There are two prevailing forms commonly in use—one in which the needle is made pointed at one or both ends to read directly upon a divided circle fixed upon the instrument, and the other form in which it is made to carry and to direct a divided circle by its magnetic force. Fig. 26.—Broad needle. Larger image Fig. 27.—Edge-bar needle. Larger image The magnetism which gives directive force to the needle has been found by experiment to reside in every separate part 117.—Magnetic needles are generally made in the form of flat bars, which are balanced upon a standing point falling into a cup which forms the centre. When the greatest section of the bar is placed horizontally it is termed a broad needle, as shown Fig. 26. This may be made of the lozenge form shown, or be parallel throughout. When the greatest section is placed vertically it is termed an edge-bar needle, as shown Fig. 27. The north pointing end of the broad needle is commonly tempered dark blue, or has a deep cut across it, if the needle is left open. This is not necessary if it carries a ring. The edge-bar is generally used where it is required to read into a fixed circle of division, in which case its ends are brought to fine knife-edges. 118.—From the difficulty of reading a sharp point in bright metal against the black line of a divided circle, the author occasionally makes one point of the needle with a fine cut, sawn vertically for a short distance from its end, so as to form a kind of split which is afterwards closed, so that it presents the appearance of a fine black line of the same character as the divisions into which it reads. With this, as shown Fig. 28, the reading is found to be much more easy. The point is also more readily adjusted by grinding, as the end of the needle being broad, less care is necessary to avoid reducing it so much that it may leave the interior of the circle Fig. 28.—Author's plan of needle reading. Larger image 119.—In the Manufacture of the Needle it should be made of the finest cutler's cast steel, or, better still, of steel containing 3 per cent. of tungsten. If not left in a parallel strip as it is drawn or rolled, it should be brought as nearly to its form as possible by forging at a low heat. The steel should not be over-heated for hardening. It should be hardened in cold water or oil, and be tempered afterwards down to a very pale straw-colour—in fact, the temper colour should only just appear. Long needles may have the temper sufficiently lowered at the centre to set them approximately straight during the tempering; but the temper should not be lowered even in the centre below a pale blue, spring temper. After tempering, the setting and working up to balance is best done by grinding, and for the final adjustment, by stoning with Water-of-Ayr stone. 120.—Magnetisation of the Needle may be performed in many ways by means of a permanent magnet or an electro-magnet, or electrically by means of a solenoid. When the magnetism is induced from another magnet it is only important that the properly hardened needle should be regularly and equally magnetised over its surface by pressure upon it of the proper poles of the inducing magnet—that is, that the north pole of the magnet should induce magnetism in the southern half of the needle only; and the south pole in the northern half only. 121.—Method of Magnetisation by Single-touch.—This method is more generally applied to touching up needles than magnetising them at first. The northern pole of a strong permanent magnet is stroked down the southern end of the needle from its centre to its end three times on one side of the needle. The needle is then turned round, and the northern end is stroked down in like manner with the southern pole. The needle is then turned over, and the process is repeated on the other side. This may be done a second time and the edges of the needle be stroked down also. 122.—Method with both Poles.—In this process the needle is held down firmly with pegs on a board, and a strong horse-shoe magnet with rather close poles is laid on the bare needle without its cap, in a manner that both terminals press upon it. It is then drawn backwards and forwards from end to end of the needle several times, lifting the magnet finally from about the centre. The process is then repeated on the opposite side of the needle and its edges. Fig. 29.—Divided-touch magnetisation. Larger image 123.—Method of Divided-touch is a somewhat quicker process, which does not entail removing the cap, the general plan of which is shown in the engraving below. The poles of the magnets, or one of them, is marked. Two good straight bar magnets are used. The needle is fixed down on a board and the poles of the two magnets are laid upon it at an angle of about 30°, applying one north or marked pole, and one south or unmarked pole. The magnets are then drawn apart 124.—It is found that the needle is magnetised a little more quickly if it is laid upon a strong magnetised bar during magnetising, or upon the ends of two bars, as shown in the engraving, Fig. 29, or on the two ends of a wide horse-shoe magnet. 125.—Needles are now more generally magnetised electrically by placing them in a solenoid or coil of stout insulated copper wire through which a strong direct current is passing from a dynamo or powerful battery. This method is employed in the best shops. The touch system above described is convenient for the profession for remagnetising a needle when weak, as a horse-shoe magnet at small cost may be kept for the purpose. It is generally used in small shops, as being at all times ready to hand, less expensive, and sufficient to ensure saturation if it is skilfully done. 126.—With every care in the manufacture of the needle there remains a little difference in the qualities of needles which are apparently otherwise identical. Little local differences in the quality of the steel, slight over-crystallisation from over-heating in hardening or unequal tempering, or unequal magnetising, are liable to form weak parts, or even what are termed consequent points. These are points in which the magnet possesses a reversal of its general longitudinal polarity. This can be made quite evident by experiment, as it is possible to make a needle not only with poles at each end, but with 127.—Mounting of the Needle.—The needle for a surveying instrument has a female centre upon which it is suspended. The centre, termed technically cap, is generally formed of a hard precious stone, agate, chrysolite, ruby or sapphire, the latter being best, simply from the high polish it attains in grinding out with diamond dust. Rubies and sapphires are like minerals, except in the colour, which varies very much; the off-colour stones, which are of small value for jewellery, are used for scientific purposes. The cap is mounted in a brass or aluminium cell made as light as possible for sufficient stability. The needle is supported upon a hardened steel point, upon which it is perfectly balanced. The base of the point is tempered down to a low degree in order to admit a certain amount of bending to counteract the slight warping which generally occurs in the hardening. 128.—Correction of Errors.—The needle, after it is mounted, although in balance may not have the steel placed symmetrically about its axis through slight curvature, unequal thickness about the cap, or otherwise, so that the magnetic direction is not perfectly linear between the points and the centre. If the points and centre are not magnetically linear, the correction for declination, which will be presently considered, cannot be made accurately. On this account it is better for the manufacturer to mount the needle on a slate bed with two sliding heads that may be brought up to the points of the needle. The heads have upon their upper surfaces lines drawn perfectly linear with the centre point of suspension of the needle, and a few lateral divisions to these lines for determining errors. On this bed the needle is placed upon the centre point to be examined how nearly its reading points are true with the axis. The error being Fig. 30.—Section of mounted needle. Larger image 129.—Lifting the Needle.—The needle of a surveying instrument should never be supported upon its centre except for the time it is in use for observation, as a fine steel point against a hard stone must, by any jar in conveyance from place to place, receive a certain amount of abrasion that will make it duller. For this reason a lift for the needle is always provided in scientific instruments. In the engraving, Fig. 30, an edge-bar needle is shown in section with its lift. The lift is made in the form of a bent lever, whose fulcrum is upon the bottom of the box. On the left-hand side of the broken line at B the needle is shown lifted. On the right-hand side A the needle is shown at its position for use, floating just slightly above the divided circle D. The pressure of the milled-head screw C depresses the bent lever or lift on the bottom of the box and thereby raises the point under the centre of the needle. This point has a hollow cone formed upon it which fits over the standing-point to keep the lift in position. The cone fits externally into the cap to lift the needle vertically. The screw C should always be clamped down when the needle is out of use. In place of the screw a wedge shaped sliding piece is sometimes fixed inside the compass-box, which is moved by a stud projecting through the outer case. Another plan of raising the lift is by a cam, or 130.—The Inclination or Dip of the Needle is the position a needle balanced level upon a free centre before magnetisation takes in the vertical plane after magnetisation. This inclination or dip varies in different parts of the globe, and at different times. At the present time at Greenwich (Jan., 1914) the angle is 66° 50' from the horizontal. It is uniformly nearly nil at the equator, and increases until over one of the magnetic poles, where it becomes vertical. There are two magnetic poles in the northern hemisphere active in directing the needle, one in Siberia, but the most active is about Melville Island; also two in the southern hemisphere, which are supposed to be nearly together, but the exact positions of which are not ascertained. As we require only the horizontal component in surveying and not the dip, it is necessary to balance the needle in opposition to the direction of the dip until it keeps in a horizontal position. This may be done by making the needle lighter on the dip side—that is, the northern in this hemisphere. But the plan adopted in all scientific instruments is to place a rider over the needle, as shown Fig. 30 under B. This clips the needle sufficiently to hold it firmly to its place, and yet is loose enough to be moved by the fingers to balance. The rider has to be shifted when the instrument is taken into a country where the dip is different from its position at home. When a needle is taken abroad without any rider, it may be balanced by means of a little sealing-wax placed upon its uptending end. 131.—To get at the needle for suppression of dip when it is placed in the compass-box, it is necessary to raise the spring 132.—The Declination of the Needle, that is, its variation in pointing in a true northernly and southernly direction, is necessary to be known and considered by the surveyor where the needle is used, both in relation to the locality and to the time, as this declination not only varies in different countries but also from year to year. For instance, this year (Jan., 1914) it points 15° 12' West at Greenwich. The following chart, Fig. 31, gives the declination variation for 1914. The whole system of declination lines is now moving westward at the rate of about seven minutes per annum, but the rate varies slightly and from year to year. The declination lines, independently of correction, which will be presently considered, may not be exactly represented by the symmetrically curved lines shown in the figure. There are small local deflections from the theoretical curves here given, which are permanent and need local consideration when using the needle for obtaining very correct bearing. These have been ably considered by Professor RÜcker and Dr. Thorp, but the subject is too complicated to be entered upon here, except for this note of observation. 133.—For new countries, where the needle often becomes most important from the impossibility of tying up lines by direct observation through forests and other obstructions, Fig. 31.—Magnetic and Greenwich time chart for Great Britain, 1914. Larger image 134.—The Magnetic Variation of Declination in Time, becomes important in reference to old plans in which the magnetic north of the period has been plotted for the true north very much to the pecuniary advantage of the legal
It will be seen by the above table that the needle pointed due north in 1663, that it attained its greatest western declination in 1818, and that it is now losing its westerly declination at the rate of about 7' annually. 135.—Annual Variation.—The declination is subject also to a small annual variation which is greatest about spring time, diminishes towards the summer solstice, and increases again during the following nine months. It varies at different periods, and seldom exceeds 16' of arc. 136.—Declination Correction to true north may be made for the compass by observation in this hemisphere of the pole star, which is practically due north in January at 6 p.m., February at 4 a.m., March at 2 a.m., April at midnight, May at 10 p.m., August at 4 a.m., September at 2 a.m., October at midnight, November at 10 p.m., December at 8 p.m. Most surveying instruments, except the transit theodolite, are not made convenient for this observation. More generally observations of the position of the sun may be made where a sun-glass is provided to the telescope of the theodolite, Fig. 19, SG, page 45, with the aid of a chronometer or a good watch. For this observation we may remember that the sun is true south at twelve o'clock on the 16th April, 15th June, 1st September, and 25th December. The following table may be useful for some intermediate times to show how much the chronometer
137.—As variation in time of southing is from fourteen minutes fast to sixteen slow, or a difference of thirty minutes, correction becomes important, as the sun passes over 7½° in this period. In these observations the diaphragm lines, webs, or points must bisect the sun's disc. This is done more exactly by taking the mean positions of the sun's eastern and western limbs or its semi-diameter, which is given for every day of the year in the Nautical Almanac. 138.—The Compass-box.—The needle, as it is generally mounted for the theodolite, mining-dial, and many other instruments, reads into a divided circle of 360°. The circle is raised up from the bottom of the compass-box to the height of the top of the needle, as shown in section Fig. 30, D, and is generally silver-plated. The bottom of the compass-box is sometimes divided with a compass-rose giving the points N. E. S. W. The E. and W. in some cases are reversed from their natural directive positions from the centre of the box, so as to read the letter indicating the point nearest to the division instead of that opposite to it. In modern surveying instruments, however, no regard is paid to the points of the compass, north being 0°, east 90°, south 180°, west 270°. 139.—In the manufacture of the compass-box very great 140.—In the construction of the compass-box the author has found the most certain method of getting the divisions correct with the centre is to make the division directly from the standing-point of the compass, and not to try to get this point correct to the divisions afterwards. The standing-point may be fixed directly to the box by screwing, or be attached to a brass plate before fixing. It is adjusted to the compass-box by bending until the needle turns freely, but at the same time nearly touches the circle. The needle is then removed and the circle is divided with the point as its centre. Where the divisions read to the point of the needle, or to a line upon it without a magnifier, the divisions of the circle may be made directly upon the lathe by a lever to the slide-rest if the lathe has a well-divided headstock. When the divisions are magnified and require great accuracy, or where a floating ring is used upon the needle, the circle should be divided upon the dividing engine, which will be described further on, the centre used being still the point or pivot on the bottom of the case, from which the divisions are to be made radially. 141.—Preservation of the Magnetism in Needles.—It is most important that the magnetism of the needle, particularly in mining-dials where so much depends upon it, should be preserved to near saturation in order to secure certain direction in opposition to the friction of the centre, necessarily always present. This is often much neglected from carelessness, or want of knowledge of the principles of magnetic action. In the first place we know that a bar of soft iron, possessing no 142.—A valuable precaution for a needle in constant wear is occasionally, say twice a year, or much oftener if it is used in a dusty mine, to take it out of its box and wipe out the cap with the point of a small sable brush. The standing-point may at the same time be sharpened if necessary by gently rubbing it all round with a slip of oiled Arkansas stone at its former pointing angle. The sharpness of a needle is easily ascertained by sliding the thumb-nail over the point at an angle of about 30° to it. If the point sticks and holds the nail, it is sharp; if it glides upon it, it is dull. The author has often had compasses of various kinds sent to him for remagnetisation whose only fault has been dulness of centre. 143.—Ring Compasses.—In modern theodolites, levels and prismatic compasses, the magnetic needle carries a light 144.—Mariners' Compasses, and an inexpensive class of prismatic compasses, are made with a paper disc in place of the ring above described answering the same purposes. The paper disc is generally made in two thicknesses with a thin sheet of talc placed between them. Mariners' compasses have frequently the divisions painted directly upon talc for transparency by lighting from beneath, also for general lightness combined with stiffness. 145.—The reading of mariners' compasses, and the compasses on levels where the needle carries a divided ring, is taken from a line drawn vertically up the inside of the box or a 146.—Trough Compass, sometimes termed a long compass. Where an instrument possesses a double vertical axis and a divided circle, as the theodolite, the division of the circle may take the place of the divided ring of the compass and save the repetition of the graduation, at the same time the needle may often be made longer, as the bulk of the compass-box is proportionately less. In fact in all cases where the magnetic north only is required the trough compass is to be preferred. The ordinary construction of this compass is in the form of a narrow box, Fig. 32, A representing a plan, and B a parallel section taken through it horizontally. About 10° are graduated on each side of the meridian line, aa being adjusting screws to bring the scale true with the needle. Fig. 32.—Trough compass for attachment to an instrument. Larger image 147.—Magnification of Reading.—With the trough compass it is very common to have some form of microscope for reading the needle more exactly. This may be done by a Ramsden eye-piece being placed directly over the needle, as is common in some German instruments. A much more convenient plan for certain instruments is to read the needle longitudinally. This is generally done by means of a transparent scale being placed across the end of the needle which is divided upon glass or horn. This may read to either the near or distant point of the needle. A very good form of Fig. 33.—Needle with reader. Fig. 34.—Scale at G. Larger image 148.—The Prismatic Compass, shown Fig. 35, was invented by Charles August Schmalcalder in 1812. It is the most convenient portable instrument for reading magnetic bearings. Angles may be taken with great rapidity within about 15' of arc by holding the instrument in the hand, or perhaps within 5' if the instrument is of 4 to 6 inches diameter and placed on a stand. It is a most valuable instrument for filling in close details, such as may occur among buildings, trees, etc., after the principal points have been laid down from observations taken with the theodolite. The principles of the reflection of a prism were discussed, art. 55, Fig. 3, p. 29. Fig. 35.—Ordinary prismatic compass. Fig. 36.—Section of the same, but with mirror. Larger image 149.—Prismatic Compasses are made from 2½ to 6 inches in diameter. The compass needle is sometimes made to 150.—Additional Parts commonly provided with the prismatic compass are a mirror and sunshades, shown only in section Fig. 36. The mirror M is carried in a frame attached with a sliding piece to the window, upon which it can be placed either upwards or downwards. It is jointed with a hinge so as to be set at any angle. By reflection from the mirror, bearings in azimuth are taken much above or below the horizontal plane. Sun-glasses are also provided in front of the prism, which are used for taking the sun's place either with or without the mirror, a single sun-glass being also used very comfortably for working towards the sun at all times. The sun-glasses, which are simply small, dark-coloured glass circles in frames, are not shown in the engraving. 151.—To Prepare to take Observations with the Prismatic Compass. After the window and prism are opened out, the prism is adjusted to read the divided ring sharply when 152.—In Using the Prismatic Compass, the compass-box is held with the thumb of the right hand under the prism at SL and the forefinger upon the stud S. The object which it is desired to observe is sighted through the slit SS, cutting the left-hand side of the hair in the window SV, while the division which comes opposite the reading point at its edge by the reflection from the prism is noted. The ring when free oscillates for a time, but is easily brought to rest for reading by gently pressing the pin S upon which the forefinger is placed. 153.—Where objects are observed for taking their bearings above the horizontal plane, the length of the window will be sufficient to take in a vertical angle of 20° to 30°; but for such altitudes it is necessary to take very great care that the compass is held level, to get magnetic angles even approximately true. Below the horizon, angles can be obtained with somewhat greater certainty by means of reflections from the mirror. Altogether, except for taking nearly horizontal angles, or for very close work in filling in after the theodolite, it is much better to have the prismatic compass mounted upon a tripod stand. With a stand, where the angle in azimuth is much above or below the horizontal plane, it is better to have a small glass level, described further on, art. 181, to place across the compass when setting it up. If the compass ring is very carefully balanced across 90° to 270° two bright wire points may be placed inside the compass-box, level with the compass ring, which will answer for the cross levelling. 154.—Stands.—The author has made a very simple and inexpensive tripod stand for the prismatic compass, the head of which consists of a ball and socket only, clamped by a large Fig. 37.—Improved prismatic compass stand. Larger image Fig. 38.—Hutchinson's prismatic compass. Larger image 155.—Hutchinson's Prismatic Compass, Fig. 38, is now very generally used by military men. In this compass the metal cover is fixed on the top of the compass-box, and a glazed opening is placed in the cover, occupying about one-eighth of its area, near the prism. This opening gives sufficient light to the compass card to permit it to be easily read, and the loose cover is dispensed with; besides which, the cover being fixed, this, as well as the whole instrument, may be made much lighter, while retaining equal rigidity for wear. This compass is not fitted with shade and mirror arrangements as 156.—Captain Burnier's Military Compass.—This portable compass is more generally used on the Continent than other forms. It is generally combined with a clinometer, therefore the illustration is deferred, seq. with clinometers. The compass ring is set up vertical to the plane of the needle, and is read by an index point by means of a cylindrical lens. It has a pair of sights formed of a slit near the eye-piece, and a hair in the window as in the prismatic. When this instrument is held horizontally, at about a foot distance from the eye, the sight line and the index line read distinctly into the graduations of the ring. A lifter is provided to raise the compass off its centre, as with the prismatic compass, and a spring clutch to prevent continuity of oscillation. It is adapted to be set up on a plain rod stand, the socket fitting to which is held in the hand when it is used as a hand instrument. Fig. 39.—Sketching protractor for use with prismatic compass. Larger image 157.—Surveying with the Compass only.—In modern practice very little surveying is performed with the compass, except for sketch or exploring maps and filling in details, wherein the prismatic compass is useful. The magnetic needle was formerly much used for surface work, and depended upon almost entirely for underground work; but this has been found practically in many cases unsafe, from the uncertainty of magnetic variations, local and other, in the 158.—In Plotting Military Sketch Surveys from angles taken with the prismatic compass, the paper employed is ruled lightly all over with parallel lines an inch or less apart. The angles taken with the prismatic compass from 0° to 360° (northern zero) are set off with an ivory military protractor, which has lines to correspond with latitudinal lines drawn over its face at 90° to its base, so that the protractor may be placed transverse to any line drawn on the paper with its centre in any position. Particulars of this method are given in every detail in Major Jackson's Course of Military Surveying, and in my work on Drawing Instruments. The military protractor is shown Fig. 39. 159.—For making a sketch plan with the prismatic compass, a very convenient way is to use the tee-square, the upper edge of the blade of which represents magnetic east to west, the upper end of the board magnetic north and the lower end south, according to the reading of the compass. The bearings taken from any starting-point are set off on the plot by a semicircular protractor with its base resting along the tee-square. The northern angles are raised with the square at the left-hand side of the board and the southern with it at the right. The distances from the station for all bearings are measured and set off by scale. 160.—It is indifferent how many stations are taken by the prismatic compass. The measurements in any direction may continue all round an estate, and will be found fairly correct if carefully made, as the small personal errors in reading the prismatic, which may be plus or minus, tend to correct each other on the whole, and to tie up the lines. 161.—The rolling parallel rule may replace the tee-square, Figs. 40, 41, 42, 43.—Pocket magnetic compasses. Larger image 162.—Pocket Magnetic Compasses.—The subject of compasses will scarcely be complete without mention of the small pocket compasses which are so useful and universal. Several well-known forms are shown in the next illustration. The square form shown first, Fig. 40, will be found the most useful for very rough sketching. The edges may be sighted for the direction of roads, etc., or the box may be placed against a wall for taking the magnetic direction of a building. In like manner also the compass-box may be laid on a drawing and lines drawn along by the edges of the box to the magnetic directions taken. This in most cases is sufficiently accurate for architectural work, in which the exact direction is not generally thought to be important. Fig. 41 is a French form of compass with step reading level with the upper surface of the needle. Fig. 42 is an old English form with enamelled dial, with lifter under the bow of the handle. Fig. 43 is the same make in a hunter case. In this the lifter rises upon the case being closed. Fig. 44.—Trough form "Egyptian compass." Larger image 163.—The author has made a small pocket magnetic compass, which is represented in the illustration above. The needle is placed in a long box. It reads at its point into a single line when the needle is exactly parallel with the sides of the box. The lid turns up endwise. The needle is lifted by closing the box. Fig. 45.—The author's under slide for setting off variation. Larger image 164.—In a form of compass similar to the above, the author has added a thin slide to the under side of the box, by means of which the magnetic variation may be adjusted, as shown Fig. 45. This slide moves out just the amount of magnetic variation, the stud S being made concentric for this adjustment. If the slide box be made of ivory a few useful scales may be divided upon it. The compass slips into a light leather case, and is the most portable for its length of needle of any compass made. The edges of the box are used as directing lines, as above described for the square form. The Fig. 46.—Barker's luminous compass. Larger image 165.—Barker's Luminous Compass, with floating dial of mother-of-pearl, one-half of this being engraved with black figures and the other half painted black with the figures left white, permits magnetic direction to be observed in the dusk and by moonlight. These compasses, Fig. 46, are much used by travellers. Mr. Francis Barker has also designed a compass in which the needle carries a bar coated with luminous paint. |
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