400.—Striding Level.—For the adjustment of the transverse axis of a theodolite a very sensitive spirit level is used. This is mounted upon a bed, which may be formed of brass tubing, from the two ends of which adjustable legs descend, the ends of which are forked, the hollows of the forks forming V bearing surfaces. The V's rest upon the pivot of the axis. By reversing the striding level on the pivots the transverse axis of the telescope, or transit axis, can be readily adjusted truly perpendicular to the vertical axis. In the construction of the striding level, shown in detail in Fig. 162, the two striding standards SS are carried down from the ends of the casing tube B of the spirit level. These are adjustable: one, Fig. 164, by raising or lowering the end of the level tube by the capstan screws CC′, and the other, Fig. 163, by a lateral adjustment of the capstan screws PP′ that act upon the stud S, which is fixed upon an arm centred upon the axis of the tube. This connection is shown by dotted lines. By these two motions the standards are brought to perfect parallelism with each other for their bearing surfaces and adjustment of the crown of the bubble tube.

401.—Adjustment of the Axis for Setting it up over a Point.—Every surveyor experiences an amount of difficulty in getting the plummet to fall from the axis of the instrument exactly over a point upon the ground, or a mark upon a rock, or still more so upon a point in street paving in a town, which is necessary for exact work. It is easily set near the point, that is, within half an inch or so, by pressing or shifting the legs; but the difficulty increases as the exact point is approached, so that the setting has generally to be left at a certain state of approximation. There are a great number of schemes in use for moving the axis by adjustment of the instrument the small quantity required, without disturbing the legs of the tripod when they are firmly set down nearly correct to position. One of these would no doubt be generally applied to the theodolite, except for the reason that every means yet devised adds to its weight, and also to the expense of the instrument. A moderately simple plan, which is especially adapted to the parallel plate adjustment, is to make the lower flange of the theodolite, upon which it stands when set down off its tripod, somewhat larger and thinner. This flange, instead of being screwed directly down upon the tripod head, is placed between two ring plates, which are clamped together when the theodolite is set in position. The large hole in the centre of the ring permits movement of the lower plate of about 1 inch. Fig. 165 is an arrangement of this kind by Mr. J. Wallis. This is made entirely independent of the theodolite, and may be used or not as required. I is a screw that corresponds with the head of the tripod which takes the theodolite; T similar female screw to take the tripod head when the shifting centre is used; CC′ a box formed by screwing two tray-pieces firmly together; S clamping flange; HH′ clamp screwed into the top of box C. This has two handles by which the screw is moved to clamp when the instrument is in position. The weight of this additional part is about 3 lbs. The arrangement is particularly adapted to parallel plate adjustments.

402.—In an American plan of a transit by Messrs. Heller & Brightly, the flange is lifted by the parallel plate screws, which tighten it at the same time.[18] Messrs. Troughton & Simms have a plan of shifting the axis by means of a pair of eccentric plates, which carry the instrument in two directions nearly at right angles to each other. By this arrangement an amount of leverage is secured which produces an easier motion than that of shifting the weight of the instrument on the plans mentioned above. The author's schemes will be described as a part of his new theodolites a few pages on.

403.—Stadia Webs or Lines used for taking subtense angles by the telescope for measuring distances, which are frequently applied to theodolites, will be fully described, Chapter XII., in treating of subtense instruments generally.

404.—Solar Attachment to a Theodolite.—This appliance is an adaptation to the theodolite of the solar compass of W. A. Burt, of Michigan, which was made to replace the magnetic compass in determining a true meridian, or north and south line, by observation of the sun only. It was brought into general use in the surveys of the United States public lands. The solar compass consists mainly of three arcs of circles by which the latitude of a place, the declination of the sun, and the hour of the day can be set off. In the solar attachment to the theodolite the latitude arc is found unnecessary, as this is formed by the vertical arc of the theodolite; therefore the hour and declination arcs need only be described.

405.—The Hour Circle, Fig. 166, H is fixed upon the centre of the telescope upon a socket axis S, which is placed perpendicularly to the optical axis and to the transverse axes or pivots of the theodolite. This circle is divided to read five minutes of time, and is figured I to XII twice, or I to XXIV, the index being a fine line carried down on a plate from the lower arm of the declination arc, which is fixed to the socket S. The hour circle, when set to any reading, may be clamped to this position by means of the milled head placed over the socket M.

406.—The Declination Arc is of 5 inches radius, divided to read on the same plane with a vernier V to single minutes of arc. The vernier arm is fixed by a clamp at C, which carries tangent adjustment T. At the back of the vernier arm two spur-pieces are carried out directly from it, L and I. These are blocks of metal about 1½ by 1¼ by ¼ inches, which carry each a lens of a focus L to I, and a silver plate to be presently described, upon which the sun's image is received in one direction or the other.

407.—The Image Plate, Fig. 167, is marked with two sets of lines intersecting each other at right angles. The lines bb are termed hour lines, the lines cc equatorial lines; these lines having reference respectively to the hour of the day and the position of the sun in relation to the equator. The intervals between the lines bb and cc are just sufficient to include the circular image of the sun formed by the solar lens on the opposite end of the vernier arm. The axes of the solar lenses and corresponding image plates are placed parallel with each other, and with the direction of the vernier arm. Below the lower line c three other lines are cut at 5 minutes apart. These are useful for making allowance for refraction. The following description for the use of the instrument is partly extracted from Messrs. Gurley's manual.

408.—When the instrument is made perfectly horizontal, the equatorial lines and the opposite lenses being accurately adjusted to each other by a previous operation, the sun's position in the heavens with reference to the horizon will be defined with precision. Suppose the observation to be made at the time of one of the equinoxes; the arm R set at zero on the declination arc V; and the polar axis is placed exactly parallel to the axis of the earth. Then the motion of the arm R, if revolved on the polar axis around the hour circle H, will exactly correspond with the motion of the sun in the heavens on the given day and at the place of observation; so that if the sun's image be brought between the lines cc on the image plate in the morning it will continue in the same position, passing neither above nor below the lines as the arm is made to revolve in following the motion of the sun about the earth.

409.—In the morning as the sun rises from the horizon, the arm R will be in a position nearly at right angles to that shown in the illustration, the lens being turned towards the sun and the silver plate, on which his image is thrown, directly opposite. As the sun ascends, the arm must be moved around, until when he has reached the meridian, the graduated side of the declination arc will indicate XII on the hour circle; and the arm R, the declination arc V, and the latitude arc, that is the vertical arc of the theodolite, will be in the same plane.

As the sun declines from the meridian the arm R must be moved in the same direction, until at sunset its position will be the exact reverse of that it occupied in the morning.

410.—Allowance for Declination.—Let us now suppose the observation made when the sun has passed the equinoctial point, and when his position is affected by declination. Then, by referring to the Nautical Almanac and setting off on the arc his declination for the given day and hour, we are still able to determine his position with the same certainty as if he remained on the equator.

When the sun's declination is south, that is, from the 22nd of September to the 20th of March in each year, the arc R is turned towards the plates of the instrument in the opposite position to that shown in the engraving, using the solar lens at I, with the silver plate opposite at L.

The remainder of the year the arc is turned from the plates, and the lens at L and plates at I are employed in the position shown in the figure.

411.—When the solar compass is accurately adjusted and its plates made perfectly horizontal, the latitudes of the place and the declination of the sun for the given day and hour being also set off on their respective arcs, the image of the sun cannot be brought between the equatorial lines until the polar axis is placed in the plane of the meridian of the place, or in a position parallel to the axis of the earth. The slightest deviation from this position will cause the image to pass above or below the lines and thus discover the error.

412.—We thus, from the position of the sun in the solar system, obtain a certain direction absolutely unchangeable from which to run our lines and measure the horizontal angles required.

The transit theodolite will, without the solar compass, perform the same functions; but by means of this instrument the calculation for position is much more simple.

413.—Photographic Apparatus in Connection with the Theodolite.—The application of photographic apparatus as an accessory to surveying instruments has been tried tentatively for many years. A practical introduction to the subject was first given by M. Laussedat in a paper published in the Comptes Rendus de l'Academie des Sciences, 1859. The subject has since been well studied by many writers, and is written up extensively by Dr. E. Deville, LL.D., Surveyor-General of Canada, in a work entitled Photographic Surveying, published in Ottawa, to which we must refer the reader for full discussion of the subject. In England, Mr. J. Bridges Lee has invented a very suitable camera in which a negative glass photograph of 4½ × 3½ inches is taken, with an axis line from the shadow of a hair permanently photographed coincident with the axis to the telescope as it appears to view. At the same time degrees and subdivisions are taken on the photograph to right and left of the axial line. The edge of the magnetic circle is also photographed upon the plate, indicating clearly the bearing of the station taken by the axis line. The whole of these operations are performed at once in a perfect manner.

414.—Mr. J. Bridges Lee's photo-theodolite was made in excellent workmanship by Messrs. Troughton & Simms. The inventor has published a paper on the subject, to be had of the Society of Engineers, Westminster.

At the present time a camera is very commonly taken by a civil engineer for prospecting in new countries,—a convenient form of this will be discussed at nearly the end of this work—but it is not generally held that photography will ever offer a means of expeditious surveying, except possibly in very mountainous countries where the necessary stations for observation become difficult of approach and of clear definition. The objections to the more general adoption of photography are, otherwise, that the processes are in degree tedious, and require special skill in manipulation, and that the apparatus is heavy and expensive with sensitive glass plates for use with it.

415.—There are many cases, no doubt, where a photograph would be valuable for the exact definition of a station. To meet this case the author has made a small light camera, shown Fig. 168, giving photographs 2 × 2 inches only, with axis line from shadow of a point. The camera to be placed when required upon the telescope of a theodolite for special cases. He has lately used his patent slide for this camera that carries films which will be further described at the end of this work. The films are unbreakable, and remain sensitive many years if kept dry. The weight of this camera with its double slides and 100 films is about 1 lb. There is ample room for it in the ordinary theodolite case.