The possible movement of the centering-plate is illustrated by the dotted circles in Fig. 81, and the range of movement of the plumbbob is shown by the dotted lines H1 and H2, Fig. 80. It should also be stated that the sketch shows an arrangement for utilising this contrivance as a wall-stand, the lower plate A (Fig. 80) having four spikes provided for that purpose, or having a female screw within, A and B. It can be screwed on to the tripod-head.

Above the joints of the stock-head is the screw upon which the instrument is held; and when not in use this screw is protected

by a hat-shaped cap, but which has recently been improved by having a box-spanner attached. Fig. 83 shows the stock-head complete, Fig. 84 is the ordinary cap, and Fig. 85 is the improved cap and spanner.

We now come to the instrument proper, which consists of three parts: the parallel plates, the horizontal and the vertical limbs.

Parallel Plates.-The parallel plates are illustrated in Figs. 86 and 87, and consist of two circular plates, kept a certain distance

apart by a ball-and-socket (which forms part of the upper portion of the instrument), and four screws, B1, B2, B3, B4, placed at right

angles to each other, and called the parallel screws. The upper plate is pierced with four holes, which are tapped with a female screw, in which a screw having in its centre a milled head works, but whose lower extremity rests and works upon the lower plate; and in order to prevent the upper plate revolving there is a U-shaped guard round one of the screws.

Parallel Plate Screws. The action of the parallel plates is regulated by screwing and unscrewing each pair of opposite screws. Thus, if the right end of the plate, as D', Fig. 86, is

required to be raised, then the left end c1 must be depressed, which is effected simultaneously by turning the screws B2 and в1, Fig. 87, inwards, whereby B2 is elongated and в1 shortened. If, on the other hand, it is desired to elevate at c' and depress at D', then these screws must be turned outwards, whereby B1 is elongated and B2 shortened. Similarly, B and B have to be dealt with. Fig. 88 illustrates how the screws are manipulated.

Fig. 88.

Ball and Socket Arrangement.-Referring to the ball-andsocket arrangement, it is necessary here to explain that it forms one of the most important parts of the theodolite. The lower parallel plate has a dome-shaped socket very accurately turned to receive the semi-spherical lower portion of the body-piece. The upper parallel plate has also a socket, upon which rests the shoulder of the body-piece; thus the four parallel screws serve to keep the upper and lower plates apart; and according to the elongation or shortening of each pair, so the ball-and-socket arrangement admits the elevation or depression of the upper plate as required. The object of this is to maintain the instrument in a truly horizontal position, as will be presently explained; but having by means of the four screws adjusted it level, it is necessary that they should all firmly bite the lower plate, but not too much so, otherwise the threads of the screws will be injured, and indentations will appear on the plate.

Now the body-piece before referred to is hollow, but its interior is in the form of an inverted cone, within which works a solid spindle of similar shape, both being so accurately ground to fit that the axes of the two cones may be parallel.

The Limb or Lower Plate.-The body-piece supports a circular plate (whose diameter distinguishes the particular size of the instrument), called the lower plate or limb. This plate is bevelled to an angle of about 60 deg., and has besides a graduated

scale (called the primary scale) divided into 360 deg. and subdivisions, marked upon silver. This scale reads continuously from

right to left. Fig. 89 is an elevation of an ordinary V theodolite, and shows the limb a and also on the plan (Fig. 90).

The Upper or Vernier Plate.-Working upon the limb, supported by the internal cone or vertical axis, is another circular plate of less diameter (by the width of the chamfered edge), called the vernier plate, в, which, unlike the limb, is only bevelled in two places, 180 deg. apart. At these two points, L L, Fig. 90, are what are called the verniers, for the purpose of minutely reading the subdivisions of the primary scale. Now the vernier plate is so constructed that it will freely move upon the limb, being held thereto by a washer and set-screw passing through the lower portion of the body-piece into the vertical axis of the vernier plate, so that, unless prevented, both plates can work con

centrically, one upon the other, and the lower one upon the socket of the upper parallel plate. But for the purpose of taking an

angle, it is necessary to fasten each plate according to their relative positions.

Clamps. It will be noticed in Fig. 89 that, underneath the limb, there is a loose collar marked v, terminating in two lugs, through which a screw v' passes, which being tightened (Fig. 91) clutches round the bodypiece and holds it tightly. This is called the clamping arrangement, as v' is the clamp-screw.

Fig. 91.

Tangent or Slow-motion Screw. Now we have seen that this body-piece only rests upon and works in the socket of the upper parallel plate, and the mere fact of clamping it, unless it were prevented from moving on this plate, would be of no avail. Thus a bracket, also attached to the collar, is connected with a sphere moving on a pivot from the bracket, and pierced to receive a screw w (which also pierces a sphere) working tangentally to the vertical axis. This last sphere revolves upon a spindle attached to the upper parallel plate. By this means a gradual motion is imparted to the limb, which admits of the most accurate adjustment. This latter arrangement is called the tangent or slowmotion movement, and the screw w is the tangent-screw. For the purpose of fixing and regulating the vernier-plate there is a clamping arrangement P (Figs. 89 and 90) which clutches the upper

CLAMP AND TANGENT SCREWS.

49

plate to the limb, whilst a gradual motion of the upper upon the lower plate is effected by means of the slow-motion screw Q, which works tangentially through pierced spheres attached equally to the one and the other.

The foregoing is a brief description of the ordinary clamping and slow-motion process in nearly all theodolites.

G

Troughton's Clamp and Tangent Arrangement. It is necessary, however, to mention that a very considerable improvement in this system has been introduced by Messrs. Troughton and Simms, and by their courtesy I am able to give an illustration in Fig. 92. The vertical axis a works within a collar B, having two arms. This collar has a segmental recess on the left side to secure the axis when pressed by the tumbling-piece c, which is actuated by the clamp-screw D. The slow motion is imparted by means of the spring F on the one side and the tangent-screw G on the other, and pressing against a lug E fastened to the plate below, so that as the

E

B

Fig. 92.

screw is turned the spring recedes or follows it, and the most minute movement may thus be effected. It occurred to me that this spring might in time lose its power, but the makers assure me that with ordinary fair usage this is not likely to happen. Such being so, there can be no doubt that it is an improvement upon the other method.

J

Level

Levelling the Plates. I have spoken about the necessity of having the instrument perfectly horizontal, to ensure which, there are placed upon the vernier plate, at right angles to each other, two spirit-levels (J and K, Fig. 98), each of which is parallel to one pair of screws, thus: J with в1 and в2, and K with в3 and B2; and by the manipulation of these screws the bubbles of J and K should be brought exactly to the centre of their run; this being so, if the instrument is in good order and adjustment, then the vernier plate and limb are parallel with the horizon. This is one of the most important matters to be

and

B'O

о OB2 K

Leve/

OB3

Fig. 93.

attended to, as unless the instrument is perfectly level the value of the observation is impaired.