Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

THEODOLITES—CONSTRUCTIVE DETAILS OF 5-INCH AND 6-INCH TRANSITS—SPECIAL ADDITIONAL PARTS—PLUMMETS WITH SCREW ADJUSTMENTS OF IMPROVED FORM—STRIDING LEVEL—LAMP—ADJUSTMENT OF AXIS OVER A POINT—SOLAR ATTACHMENT—PHOTOGRAPHIC ATTACHMENT.

364.—The Theodolite is the most perfect instrument for measuring both horizontal and vertical angles by the aid of a telescope and graduated circles. For the purpose of surveying, the theodolite is mostly employed to take a system of triangles upon the horizontal plane of the surface of the land, and of objects at any position in which they may be placed. When altitude angles are taken separately these are generally applied to give corrections to chain or other actual measurements upon the surface by calculation of the difference of hypotenuse and base.

365.—The theodolite in all its essential features, as differentiated from sighted compasses for taking angles, mentioned by Digges,[15] was the invention of Jonathan Sisson, a celebrated mathematical instrument maker of the beginning of the 18th century.[16] Great improvements were afterwards made in this instrument by Ramsden, who brought it up nearly to its modern efficiency by the introduction of the transit principle.[17] Later improvements in portable instruments consist in the application of the transit principle to the telescope, which was formerly applied to astronomical and the larger geodetic instruments only. Other improvements have been made more recently in constructive details.

366.—Theodolites were commonly made of two distinct types, which were originally distinguished as plain theodolites and transit theodolites. In the plain theodolite the telescope moves through an arc of about 45° upwards or downwards from the horizontal plane, but very few of these are now made compared with the number of transit theodolites in which the telescope may take a complete revolution upon its horizontal axis, so that a back and fore sight may be taken by a half revolution. This difference of construction entails a difference in the manner of mounting the telescope to correct its adjustments. In the transit the accuracy of centring and reading is easily discovered by taking a back and fore sight at a distance as equivalent to an arc of 180°, which may be read on any part of the limb by transitting the telescope, wherein the correspondence of this arc to the reading of the limb to right and left hands will detect error. With the plain theodolite the equivalent method of examination is effected by placing the telescope in Y's, as previously discussed for the Y-level, and turning it end for end on its bearings, a process liable to disturb the direction of the telescope unless special care be taken. In the following description of the details of construction of a theodolite it will be convenient to take the transit form of instrument, as this is more comprehensive, the original pattern being selected, as this may be constructed with the limited amount of tools generally found in a surveying instrument workshop.

367.—The size of a theodolite is fixed technically by the diameter of the line of division upon the horizontal circle. A 5-inch or 6-inch theodolite is the largest size that may be carried comfortably in a single case; and no great advantage is gained by having an instrument beyond this size if the work is that of the ordinary surveyor on town and county surveys. The verniers of 4- and 5-inch instruments read sharply to single minutes of arc, which is as nearly as can be plotted with any degree of certainty with an ordinary protractor reading by vernier also to minutes only; 6-inch instruments read to 30 but generally to 20 seconds. Occasionally 4-inch theodolites are selected for lightness at a sacrifice of capability and of distinct and exact reading. The following table gives the average weight of the transit theodolite illustrated on the last page:—

It will be seen that the 5-inch instrument of this class with cases and tripod, say altogether 36 lbs., is really of quite as much weight as a fairly strong man can carry through a hard day's work. The 5-inch instrument is therefore becoming more and more popular with practical civil engineers, and its performance, if of good modern work, is quite equal to the 6-inch of less than half a century ago.

368.—By giving a description in detail of a transit theodolite, the general principles of a great number of other instruments, particularly those of larger dimensions, will be included, except for certain details that the specialities of the particular instruments demand. The most convenient plan to follow in this description will be to take the structure of a 6-inch transit theodolite of common construction, as it is built up from its base, piece by piece, according to the rule of ordinary structure; where more modern theodolites vary mostly from this is in having many parts shaped out of the solid, which are screwed together in the form illustrated.

369.—The Tripod Stand of a theodolite of 6 inches and under is generally made identical with that of a level, a common form being that described for a dumpy, art. 216. The arrangement of one turn-up leg, as shown Fig. 63, is very advantageous for the theodolite if it is to be used on mountainous or even very hilly ground. For instruments exceeding 6 inches a framed stand, which will be described further on, is better. Some makers use a framed stand for a 6-inch instrument. The rigidity of the stand ought to be quite equal to that of the work in the theodolite, or a little in excess, and when this is attained it is sufficient. Where the stands of theodolites so often fail is from the defective construction of the tripod head, not at all from deficiency of timber in the tripod itself; and overloading this, in adding weight without attention to scientific construction, is worse than useless.

370.—In the following description of the transit theodolite the parallel plate setting-up arrangement is taken, as this is at the present time (1914) still in use in this country and in America. There is nevertheless great probability that it will not long continue to be so, as year by year the tribrach system, described art. 233, for levels is coming more forward, both for levels and theodolites. This tribrach system the author holds to be much more scientific, and when thoroughly understood, more simple and expeditious to work with. It is also to be recommended, as there is no possible risk of strain upon the general work of the instrument, nor risk of error from distortion of the vertical axis from strain in setting it up to adjustment. A constructive drawing of a common transit theodolite with parallel plates is shown Fig. 153, of which the following is a detailed description.

371.—The Lower Parallel Plate N.—This has a large boss-piece taken up from its central part, which forms a dome of a hollow globular section, technically termed the socket, shown at X. In the interior of the lower part N a coarse female screw is cut, of about fourteen threads to the inch, which is used to attach the instrument to its tripod.

372.—The Upper Parallel Plate is constructed as a flange from a solid boss L. This piece is generally made in gun-metal of a form as solid as possible, to resist the straining action of the parallel plate screws. The boss is prolonged downwards by a stem-piece, upon the lowest part of which a ball collar of globular section is firmly screwed. The screw is turned by means of two opposite holes, into which a powerful forked screw-driver is inserted, until it is jambed up too tightly against its shoulder to ever become loose by the ordinary use of the instrument. The ball collar fits into the socket carried up from the lower parallel plate. The whole of this globular arrangement is termed the ball and socket. The boss L of the upper parallel plate, with its stem, has a hollow conical hole through its axis, into which the body-piece, to be described, fits accurately. Upon its outer upper part an inset collar is formed which acts as a guide to the clamp K. At the outer edge of the parallel plate M′ four vertical, conical holes are made, which take socket-pieces, which are tapped as nuts to the parallel plate screws M. These socket-pieces are jambed into their holes tight home to their shoulders. The socket-pieces are made separate, both to give a greater length of female screws than the thickness of the plates, and that they may be easily restored at any time if worn loose in the threads by the action of the plate screws.

373.—The Parallel Plate Screws.—One in elevation is shown at M, with its point dotted, and one in section at M′. The four parallel plate screws are in opposite pairs, placed exactly at right angles to each other in a line passing through the vertical axis of the instrument. These are made of gun-metal about 3/8 inch in diameter, with a deep thread of about thirty-two to the inch. They require cutting on a nice steady screw-cutting lathe. The lower points of the screws are slightly domed, sufficiently only for the amount of rocking they have to take, so as to impress the lower parallel plate as little as possible. The milled heads M are placed between the parallel plates, not above, as previously described for levels. There being a constant strain upon these screws in use and by intrusion of grit from flying dust they soon become worn. After wear the threads may be recut deeper, and new socket-pieces fitted to the upper parallel plate. To prevent wear the upper parts of these screws are sometimes encased in tubes—a plan very generally adopted in America. At the foot of one of the parallel plate screws a stay-piece is fixed to the lower parallel plate, which forms a kind of ring round the screw. This prevents the parallel plates from shifting upon the axis at the ball and socket. The parallel plate screws should be without any shake or what is technically termed loss of time. They should move firmly but softly. They should support the instrument against the ball and socket upon which the whole rocks to position by their aid, but not be screwed down too tightly, as this has a tendency to disturb the axis of the instrument however solidly it may be made. Makers often have instruments in their hands for repairs in which the parallel plate screws have been deeply indented into the lower parallel plate, with the centre of the instrument permanently strained more or less.

374.—The Body-piece.—The only outward part seen in elevation of this is shown at T: it is shown in section T′. This piece carries the limb of the instrument SS′ by a centred collar to which it is attached by screws. About the centre of the body-piece an inset collar is formed to take the clamp KK which bites upon it. The lower outer part of the body-piece forms a conical fitting in the boss of the upper parallel plate L. The interior is a hollow conical axis. The body-piece is generally made of hard gun-metal. The greatest possible care is required in its manufacture, art. 21. The interior and exterior should be perfectly concentric at every part. Much of the value of the instrument depends upon the perfection of the work in this piece.

375.—Axis Collar Clamp K has been already described, art. 349, and is illustrated in Fig. 146, which is taken from a theodolite, so that only specialities in relation to the instrument Fig. 153 need be noted. This clamp surrounds the body-piece and clamps it by means of the screw K shown on the left hand. The clamp is connected with the upper parallel plate through the tangent screw, the head of which is shown at P, so that when the screw K is tightened the parts L and T are fixed together, except that a slow motion can be given to these parts by the tangent screw P. By this clamp and tangent arrangement the whole of the upper part of the instrument is rendered free to revolve, to bring the instrument to bearing when the clamp is loosened, the final adjustment being secured after clamping by the tangent screw. It is this part of the instrument which is used after setting it up to bring the magnetic needle true to magnetic north, or otherwise to direct the telescope to any established distant mark, object, or star that may be fixed for the zero or other index point of the horizontal circle, to which all readings from its position are referred.

376.—The Central Vertical Axis is shown only in half section at Z. This is made uniformly of bell-metal, in the form of a truncated cone, extending from the horizontal circle plate S to the interior of the socket N. Its fitting surfaces are at the two ends of the cone, extending about half an inch, the central part being chambered back. At the upper part a pin-piece centres the vernier plate, to which it is attached by a wide collar with three or four screws. A square shoulder rests with weight only just sufficient to support the instrument upon the body-piece. This part has to be so adjusted that the axis perfectly fits and yet moves freely. A square-hole collar and screw are fixed on the lower end of the axis, just to touch the socket of the body-piece, so as to secure the axis in its position when the instrument is lifted. An eye or a hook is fixed into the screw at the lower end to take the cord of the plummet used for fixing the instrument over a definite point on the ground. This is not shown in the engraving.

377.—The axis of an ordinary theodolite is made the weakest part. It is generally considered in the trade right for it to be so, as in case of accident no other part of the vertical axis system is likely to be deranged; and this is the easiest part to replace, being, as it were, independent of other fittings. Whether this should be taken cum grano salis is a question; at any rate with the axis weak it is not policy to load the upper part of the instrument with metal—which in places at least, is generally made ten times as strong as the axis—when the instrument has to be carried about by a person over his shoulder. Some suggestions will be made on this point hereafter.

378.—The Horizontal or Lower Plate or Limb.—Sometimes the whole of the piece SS′ is termed the limb, but more generally this word is applied to the divided part only. This plate is of brass, and is attached to the body-piece by screws. The outer rim, which is somewhat triangular section, is undercut upon the inner side of its lower surface to support the clamp-piece, the outer edge being turned to a fillet to take the clamp which is rebated to fit it. The upper surface of the rim, or the limb proper, is turned to the frustum of a cone of about 45°. This part is covered with silver, which is beaten out to the conical form and soldered down upon it, and afterwards turned to true form. The dividing has been discussed in the last chapter.

The 6-inch instrument is generally divided to 20′, but sometimes to 30′, and the vernier reads to 20″ or 30″. The figuring is from 0 to 360, right to left, taken facing the instrument.

379.—The Vernier Plate is shown in section under P′. The vernier from which it is named is shown at VV′, Fig. 155. The vernier plate is carried from the central axis and forms the foundation for all the superstructure. The upper and lower plates are left very free where they are brought together, the verniers being generally sprung down just to gently touch the limb. The vernier surface is let down some distance into its plate for protection. The reading of the vernier has been discussed in the last chapter.

380.—It may be particularly noted, as already stated, that the central axis and the body-piece are attached to the vernier and horizontal plates by screws. This plan might strike one as being unsound: it is not really so, the reason for this construction being that these axes are, or should be, of bell-metal, and that this metal being very hard and brittle it would not be so easily worked, or so serviceable as brass for the limb and vernier plate, neither would there be means of correcting errors which generally occur both in the workmanship and in the dividing of this delicate part. The adjustment for fixing the limb and vernier plate, technically called centring, in particular requires considerable technical skill. It is generally performed by the divider, who is a specially intelligent artisan. In the author's improved theodolite, to be described further on, the axis is in one casting with the standard; but in this case the construction is different, the axis being made larger and the whole body being in a special gun-metal which approaches bell-metal in hardness.

381.—The vernier plate carries the ball nut of the tangent screw, shown at Fig. 153J. The general arrangement may be seen by the section, but is more fully described art. 347. One thing is important in this screw, viz., that it should range without strain quite parallel with the plates, so as not to give the slightest tendency to elevate or depress the edge upon which it is placed during motion in any part of its thread. The clamp is sometimes placed between the plates.

382.—The Compass-box.—The general construction of this is shown, Fig. 155, W. In the transit theodolite it is fixed firmly by screws to the vernier plate and is made to form a steadying piece to the A-frames C′ C″ which support the upper part of the instrument. For this purpose the compass-box is made as a solid casting in brass, which is much stiffened by the raised step which forms the divided circle. Four solid lugs in the same casting project from the rim of the compass, and form stiffening pieces between the lower parts of the A-frames; these are secured to the lugs by four screws, one of which is shown, Fig. 153, at a. The lug screws hold the whole superstructure together quite independently of the vernier plate, to which it is afterwards firmly fixed. The compass needle is lifted by means of a milled head, just inside one of the standards, not shown. For a general description of the compass-box see art. 138. The vernier plate carries two or more verniers. The verniers are read by a pair of microscopes, Fig. 155 MM′ placed one on each end of a radial arm N having its axis of motion upon a large collar of the vertical axis. By this plan, when one microscope is set to read by the coincidence of lines upon one of the verniers, the other microscope on the other arm or arms will be set also in like position over the other vernier or verniers.

The verniers are adjusted ready for reading when the telescope is accurately directed upon any object of which it is desired to ascertain the angular position in relation to magnetic north, or a definite object. The vernier plate also carries a spirit level at O, Fig. 153, which is adjustable by a pair of capstan-headed screws.

383.—The Standards or A-Frames, shown C′ C″ Fig. 155, are solid castings in brass of about 7 inches in height. They are set up upon the vernier plate, to which they are attached by four stout screws, as also by cross screws to the compass as stated. This renders the superstructure of the transit as firm as may be in a built-up construction. Upon the front of one of the standards a spirit level, Figs. 153, 155, I, is placed adjustable by two capstan screws. This level, and one shown Fig. 153 at 0 on the vernier plate are used entirely in setting up the instrument; and being placed at right angles to each other, are a means of making the vernier plate quite level. Upon the inside of each of the standards, at about 2 inches from the vernier plate, a clip-piece, Figs. 154, 155, P is secured by two screws. This takes the clipping screws, Fig. 154 HH′ to be described. At the top of the standards two V's are formed, upon which the transit axis rests. One of these is cut out of the solid casting. The other as shown in half section Fig. 155 c is formed as a parallel sliding piece with the V at the top placed in a vertical slot formed in the standard. This sliding piece has a screwed stem continued from its lower surface that passes through a vertical hole at the top of the A-frame, which is formed here as a cross-piece. Upon the screw two capstan nuts are placed, one on each side of the cross-piece, Fig. 155 xx′; these permit the adjustment of this in height so as to get the transit axis perfectly horizontal when the vertical axis is perfectly perpendicular to the horizon. The sliding piece is covered by plates back and front to render it firm in its position. The transit axis in practice is adjusted with a striding level which will be described presently.

With the author's theodolites from 6 inches downwards the old-fashioned adjustment to one upright for levelling the horizontal axis has been dispensed with for many years, and is only fitted if specially ordered, as it has been found to be a frequent source of error. Long experience has proved beyond doubt that the fewer adjustments there are, and the more parts that can be fashioned from the solid metal correctly, the longer will the instrument keep in adjustment. Should there ever be any wear on either of the V's a few strokes with a piece of very fine emery paper upon the opposite one will put it right in half the time that it could be corrected with the old-fashioned adjustable V, and no amount of vibration can alter it as with the adjusting screws.

An axis cover cap bb′ is placed on the top of each standard. The cap is screwed down at one end with a cut screw and collar. The screw is used for adjustment to gentle pressure on the axis. The second screw is a milled head EE′. Under this screw the cap is slotted out to one side, and turns on the cut screw as an axis to open the cap without removing its milled-head screw, so that the telescope can be lifted out to turn its face to the opposite side of the instrument. In the under side of the centre of the cap a cell is bored out, into which a small cork is fitted, which produces, when the cap is clamped down, a soft elastic pressure on the axis.

384.—The Transit Axis which supports the telescope rests at its ends upon two trunnions, Figs. 154, 155 AA′, technically called pivots, in the V's of the standards already described. The pivots are turned as true as possible, and afterwards ground to exactly equal size in a collar, so that they may be reversed end for end in their bearings without changing the linear direction of the transit axis, except by the little difference of pressure that one end of the axis imposes by the weight of the vertical circle and its attachments being eccentric. In larger instruments this difference of weight is counterbalanced, as shown in dotted lines at p, Fig. 155. The centre of the transit axis is formed into a collar e of about 1¼ inches in width, which exactly fits the outer tube of the telescope, and to which it is fixed with soft solder. The collar is directly connected with and supports a flange f. Upon this flange the vertical circle FF is fixed by three or four screws.

385.—In front of the vertical circle a flanged collar-piece carries the vertical vernier frame VV′, Fig. 154, centred upon it. The vernier frame is attached by three screws to the clipping arm to be described, and in front of this the vertical microscope arms are centred. These carry two readers U, Fig. 155, exactly similar to those which read upon the horizontal circle, and they are similarly centred, so that by setting one, the other is set at exactly 180° from it. In front of the centre of the microscope arms on the transit axis, an axis collar-piece j is attached by three screws cut directly into the axis. This collar and one at the other end of the axis A′, turned out of the solid, are nicely fitted to the opening between the standards to prevent lateral displacement of the axis.

386.—The Clips.—The clipping arm, which is centred on the transit axis and attached to the verniers, is shown Fig. 154 BB′B′. It is fitted to move freely on its axis at A, so as to permit unrestrained motion of the telescope. A milled-head clamping screw with clamp, Fig. 155, K, and the same partly cut away to show the slot in which it works, are shown at K′ Fig. 154. This is used to fix the verniers stationary on the circle, except for the adjustment by the tangent screw G′, which has its collar attached to the clipping arm, and its ball nut attached to the clamp at D when using the telescope for levelling. This clamp and tangent sets the vernier to zero on the circle. It is also used in setting the telescope before angles of altitude or depression can be measured. The clipping screws HH′ are used to bring the principal bubble B, Fig. 153, on the top of the telescope to the centre of its run after the verniers have been brought to zero by means of the clamp and tangent screws. The clipping screws hold the clips, Fig. 155, P or P′ to the one standard or the other. The whole of the vertical adjustment is exactly equivalent to that already described for the horizontal motion, except that it is placed in the vertical plane.

387.—The Vertical Circle, Figs. 154, 155, F is carried by four arms from a central boss attached firmly by screws to the transit axis. It is grooved at the edge to take the clamp-piece. The silver is inlaid in this circle in the manner shown Fig. 117. The vernier is read upon the circle on the plan shown Fig. 127. The circle is divided generally to half degrees or 20′, and is figured 0 to the horizontal with 90° upwards and downwards. The zero lines are made directly coincident with the optical axis of the telescope when it is level. The vernier reads to half minutes or 20″, in either direction, the rising arc above the level datum being considered as plus, the falling arc as minus.

388.—On the outer edge of the circle or at the back a scale of difference of hypotenuse and base reads to a line on a fiducial edge upon a part of the clip BB′, Fig. 154, at N. This scale is calculated for decimal quantities, and gives the percentage number of links, feet, or metres to be deducted from the chain measurements upon the ground line to give the horizontal distance corresponding to the angle of inclination at which the telescope is set for observation.

389.—The Telescope, Fig. 153, DD′ has been described art. 94. Its general construction is also shown in partial sections in the figure. Its body tube passes through the transit axis in which it is soldered.

390.—The Principal Level Tube is generally mounted on the telescope upon two stiff screws which rise from plates attached to the telescope body by pairs of screws. Each level screw has a pair of capstan nuts. The level is mounted in a brass tube with stop-pieces at the ends, each of which carries a tenon with a hole in its centre through which the level screw passes to be clamped top and bottom by the capstan nuts. These nuts give adjustment to the level, so that the centre of its inner upper surface may be placed parallel with the optical axis of the telescope.

391.—Until 1898 the author was unwilling to attempt to remodel the old form of transit theodolite, believing the 4-screw adjustment would soon become a method of the past, but as a small demand continued from the Colonies and United States for this form of instrument he felt bound to make it of more solid construction to bring it somewhat up to date. The illustration shown, Fig. 156, is of an instrument, following in construction the transit theodolite already described in many details, the marked exception being that the standards are in one casting with the compass-box and axis, these being entirely shaped out in the solid metal. The upper parallel plate is of special design, being far stronger, yet lighter, and gives a much longer bearing to the levelling screws. The lower parallel plate is also shaped with three feet so that the instrument may be set up without its stand when required. It has also modern spring tangent adjustments with covered screws. The limb is covered, and the readers are jointed across the axis to turn up without separation. It has a floating aluminium compass read by a microscope, so that the instrument, except in the four-screw arrangement for setting up, embraces many modern improvements formerly applied only to special high-class theodolites. The improved construction permits greater rigidity with fifteen per cent. less weight.

Of later years, however, the demand for four-screw levelling instruments has been maintained, especially from Canada, owing to the influence of the American school of teaching, and in consequence all the author's improved theodolites are fitted with either three- or four-screw levelling, whichever is desired. It is a strange fact, however, that with all the American makers, although they list all their ordinary instruments with four-screw levelling, their refined ones, which they term "precision" instruments, will be found with three-screw levelling.

392.—Detached parts of a Theodolite.—The Plummet supplied with the theodolite is made to hang from a hook under the centre of the axis of the instrument, the cord, which is of soft silk, being looped or knotted to hold in the hook. The lower end of the plummet is brought to a point which, when in use, falls directly under the vertical centre of the instrument upon the surface of the ground. In Fig. 157 the screw and plummet are shown detached. The cord C is attached to the plummet by passing it through a hole in the milled-head screw S at the top of the plummet, and by making a knot K in the cord. Fig. 160 shows an ingenious ring plummet recently invented and patented by Mr. W. H. Shortt, A.M. I.C.E. The chief object was the production of a plumb bob whose plumbing point should be situated at, or very close to, the centre of oscillation in order that the position of the point might be unaffected by oscillation of the bob itself, apart from any swing which it might have about the point of suspension of the string. A further object was to shape the bob so that a person holding the string, or standing close to it when attached to an instrument and looking down at the bob, should be able to see readily the exact position of the plumbing point.

These objects have been attained by making the bob in the form of a ring, so that the centre of oscillation which lies in the centre of the ring can itself be used as the plumbing point, since it can be readily seen and indicated by the extremities of pointers projecting towards the centre from the inside of the ring.

A great advantage of this bob is that when plumbing on to a flat surface it does not fall over when lowered, but may be allowed to actually lie on the surface while the position of the point is being marked. Also it can best be steadied by lowering into contact with the ground and raising again.

The plumbing pointers are largely protected from injury when the bob is in use, and when not in use the suspension string can be wound diametrically across the bob, in recesses provided for the purpose, thus completely protecting the points.

393.—The Loop.—It is somewhat difficult in the ordinary way to adjust the plummet to the station mark on the ground or on a peg. The cord is sometimes placed in an ivory runner fixed to the top of the cord, Fig. 159. This gives friction on the cord and permits extension and contraction of the loop for adjustment. Where the plummet has to be suspended from the instrument as well as from a hook inside the stand, which is sometimes convenient, it is better to have the runner cut out on one side. This permits easy change and it is just as firm.

394.—Messrs. Gurley Bros. of Troy, N.Y., have a good plan for shortening the plummet line. This is effected by making a reel in the plummet, which is wound by a milled head at the top of it, Fig. 158.

395.—Screw-drivers, Tommy Pins, etc.—A screw-driver and a tommy pin, the last to turn the capstan heads, are placed in the case with the theodolite. Two screw-drivers with proper handles are better, as there are small and large screws. A camel-hair brush to dust the instrument, a piece of wash-leather, a little vaseline, and a small bottle of good watch oil are also very useful. These little refinements are generally kept out to keep down the price of the instrument.

396.—Additional Parts, and Variations in Theodolites.—Illuminated Axis.—4, 5 and 6-inch transits sometimes, and larger instruments always, have the transit axis bored on one side through to the interior of the telescope, as shown on Fig. 155. Through the hole a small pencil of light is sent by a lamp l with a plano-convex lens front, to a lens placed in the end of the axis. This, by a slight adjustment of the lamp on its stand, focusses the light upon a small mirror placed within the telescope, which reflects its rays to the diaphragm. The lamp gives a faint light only sufficient to distinguish the webs for night and underground observations. The mirror is about 1/10 inch in diameter, and is generally mounted upon a milled head screw tapped into the trunnion band of the telescope m. The point of the screw is extended as a thin stem into the axis of the telescope, where the mirror is held by it. This arrangement permits the mirror, which is generally made of silver, but is much better of platino-iridium, to be removed for cleaning. The lamp is mounted upon a wooden stand w carried upon a slide n or upon two brass pins direct to the A-frame. The wood is employed in this case to cut off conduction of heat to the near standard from the lamp as much as possible to prevent disturbance of the axis from expansion by heating. The stand may be removed when the lamp is not required and placed in the case. In large theodolites a pair of lamps are used, that the transverse axis may not be heated more on one side than on the other.

397.—The Lamp, which is found so convenient for bringing a star or distant light to read with the webs, becomes difficult to use when the object is very faint, as the light thrown into the telescope by the lamp takes off the effect of blackness of the night sky or that of total darkness. This becomes important in taking observations of small stars, as for instance, the circumpolar stars of the southern hemisphere. In some theodolites, made first for the Sydney Government, the author placed a very small lamp to throw light upon the face of the webs only, making these appear as light lines on a black ground. The reflecting eye-piece, Fig. 20, will be found to answer very well, and this is a simple, inexpensive contrivance. Any amount of illumination desired may be thrown on the front of the diaphragm, according to the distance at which the light is held from the eye-piece: generally a very faint light only is required.

398.—The author has illuminated the webs front and back by means of a very small (one-quarter candle power) incandescent lamp, which is charged by a portable battery, or a secondary battery where a dynamo is at hand for charging it, and for countries where these cannot be renewed or where the extremes of temperature are too great for their use, he has devised a small hand dynamo for generating the current and a rheostat for controlling the power of the lamp, so that resistance may be employed to reduce the light to the faintest possible glimmer.

The electric lamp is far superior to the old oil lamp and safe to use in gaseous mines; it is far cleaner, does not give out a tithe of the heat, and may be removed from its socket and used in the hand for reading the verniers in a bad light. All the author's modern instruments that are required with illuminated axis are now fitted with electric lamps.

399.—A Trough or Long Compass, used in place of Circular Compass.—A long compass, Fig. 32, p. 74, is often applied to a theodolite, either upon the top of the telescope, or more generally and conveniently for reading under the limb. In this last case the trough needle is a separate piece, which is only attached to the limb of the theodolite by means of loop slides or bayonet fittings under the limb, when required to take a bearing. The engraving Fig. 161 shows the long compass with bayonet fittings. There are four slots, two of which are shown SS′, which fit in under the heads of round-headed, shouldered screws. The author has somewhat modified this pattern recently by making it slide into grooves.

The trough needle is generally made 5 or 6 inches long, and reads into a short scale of about 10° at each end. The divisions are best placed upon sliding fittings, so that they may be adjusted by four screws from the outside of the box—screws shown AA′. This enables the needle to be adjusted to its own axis, and also to the 0° reading of the horizontal limb of the theodolite. A slide lift to the needle is shown at L. When the same form of compass is used upon large instruments a reader is placed at each end of the needle.