vernier plate, as shown in Fig. 2, or to one pair of standards, as shown in Fig. 1. The object glass, or objective lens, of the telescope is at o; it is protected from the direct rays of the sun by the sunshade d, and is focused by the

milled thumb wheel b, which is here shown on top of the telescope, though it is commonly placed on the righthand side.

3. A spirit level L is attached to the telescope longitudinally. This consists of a glass tube, curved slightly

upwards, and nearly filled with alcohol or a mixture of alcohol and ether, the rest of the space inside being occupied by a small air bubble, called the level bubble. The tube is protected by a brass case having an opening in the top, through which the bubble can be seen. In order that the position of the bubble can be distinguished accurately, graduations are marked either on the top of the glass tube or on a small brass scale placed just above the opening. When a level bubble is in the center of its tube, as shown by the graduations, it is said to be centered. Each end of the brass case is attached to the telescope tube by means of a small stud, as shown at h, and can be raised or lowered by means of two capstan-head nuts that screw on the stud. This level is often called the attached level.

4. The graduated arc V, called the vertical limb, or vertical arc, and the vernier v', are for measuring vertical angles. The telescope can be clamped and held at any inclination to its axis of revolution by means of the clamp screw F; it can then be turned slowly on that axis by means of the tangent screw g, to which is attached a circular scale called a gradienter. In some transits, the vertical arc is attached rigidly to the transverse axis of the telescope, as shown in Fig. 1, while in others it connects by a joint that allows it to turn freely on that axis. This joint, however, can be clamped by means of a clamp screw similar to that shown at F; and then it turns with the telescope. The vernier v', by which the vertical arc is read, is attached to the standards, and in some transits is made adjustable, so that its zero mark can readily be made to coincide with the zero mark of the vertical limb when the telescope is perfectly horizontal. In some transits the vertical arc is attached to one pair of standards, and its vernier to the transverse axis of the telescope.

5. A plain transit, Fig. 2, is one that has no vertical limb nor attached level. The transit here illustrated is of a different make, and consequently of somewhat different form from that shown in Fig. 1. Corresponding parts are

designated by the same letters in both figures, that they may be readily compared. In Fig. 2 the objective o is covered by the cap c instead of by the sunshade d shown in Fig. 1. This protects the objective when the instrument is not in use.

6. The Plates.-All transits have two concentric plates rotating independently on the same axis, which is called the axis of the instrument. When the axis of the instrument is vertical, these plates rotate in a horizontal plane. The lower of the two plates carries a graduated circle, I called the horizontal limb, or horizontal circle.

Το

the upper or vernier plate, which rotates within and above the other, are attached two verniers, which travel along the graduated circle of the lower plate. By means of these plates, carrying, respectively, the graduated circle and the verniers, horizontal angles are measured. The graduated circle is entirely covered and concealed by the upper plate, except at two small openings where the verniers are attached to the upper plate. Through each opening, which is protected by glass, can be seen the vernier and a short arc of the graduated circle. The opening through which one vernier is read is shown at v, Fig. 2; the other is diametrically opposite. The verniers are usually distinguished by marking one A and the other B. The vernier plate can be clamped firmly to the lower plate by means of the clamp screw K, called the upper clamp, or vernier clamp; and by means of the screw t, called the upper tangent screw, it can be revolved slowly on the lower plate, moving the vernier along the divided circle, so that the instrument can easily be set at any given angle. In order to prevent any lost motion, the tangent screw operates against the opposing spring s.

At C is shown the compass circle, or needle circle, which is attached to the upper plate. The needle-lifting screw is shown at n. These are the same as in a compass.

7. The Centers.-The upper plate U, Fig. 3, is attached to an accurately turned and slightly conical axis or spindle Q that extends down nearly to the tripod head. In transits of the most modern construction, this axis revolves within a socket that is controlled by the leveling screws SS and about the upper portion of which revolves a socket that extends down from the lower plate, forming what is called a compound center. The centers, which control the entire instrument above the leveling screws, can be clamped against rotation by means of the clamp screw K', and the instrument can then be revolved slowly by means of the tangent screw t'. This clamp screw is called variously the lower clamp, clamp to the centers, or clamp to the lower plate, and the tangent screw is designated by

corresponding terms. The centers are connected with the plate O, sometimes called the lower leveling plate, by means of a hemispherical or ball-and-socket joint, shown at M, Fig. 3. The centers and the entire instrument above them are supported in position by the four leveling screws. which serve also to level the instrument. The plate 0 screws on the tripod head. This plate and the leveling screws, considered together, are sometimes spoken of as the leveling head.

8.

FIG. 3

Shifting Center.-The exact position of the instrument over a point is indicated by a plumb-bob suspended from a small chain or loop attached to the lower end of the centers at the central point p, Fig. 3. It will be noticed that the centers do not connect directly with the lower leveling plate O, but, by means of the hemispherical joint M, connect with a plate B, which bears against the under side of the leveling plate. By loosening the leveling screws, the plate B and the leveling screws that support the entire