CHAPTER VII.

THE ENGINEER'S TRANSIT.

THE MEASUREMENT OF ANGLES.-Several instruments differing only in power and degree of accuracy have been used for measuring the angles in triangulation.

In topographic work undertaken for military purposes, the instrument most likely to be used in measuring angles is the Engineer's Transit (Fig. 95). This is the most accurate portable field instrument for the purpose, and with its various attachments it may be used for leveling, measuring distances, determining meridians, reading bearings, fixing grades, etc.

DESCRIPTION OF THE TRANSIT.-The essential parts of the Transit, as shown in the cut, are the telescope with its axis and two supports or standards, the circular plates with their attachments, the sockets upon which the plates revolve, the leveling-head, and the tripod on which the whole instrument stands

THE TELESCOPE is from 10 to 11 inches long, firmly secured to a horizontal axis having its bearings nicely fitted in the standards, and thus enabling the telescope to be moved either up or down, or revolved completely around if desired. The object-glass D (Fig. 96) is composed of two lenses, so as to show objects without color or distortion, and is placed at the end of a slide having two bearings, one at the end of the outer tube, the other in the ring C C, suspended within the tube by four screws, twc of which are shown in the cut. The object-glass is carried out or in by a pinion working in a rack attached to the slide, and thus adjusted to the distance of objects, either near or remote, as desired.

The Eye-piece is made up of four lenses, which, beginning at the eyeend, are called respectively the eye, the field, the amplifying, and the object lenses, the whole forming a compound microscope having its focus in the plane of the cross-wire ring B B. The eye-piece is brought to its proper focus by a rack and pinion movement. This being an adjustment to suit the eye of the observer, when once made needs no further notice for the same person. Sometimes an eye-piece of two lenses only is employed; but this nverts the object seen and so has generally been discarded in American transits. Where it is desired to take vertical angles so great that the eye can not be placed under the telescope, a little cap on the end of the eye-piece is unscrewed and replaced by one containing a small prism, which reflects the image of the object at right angles and brings it to the

FIGURE 96a.

eye of the observer
(Fig. 96a). When

used on the sun, a colored glass or darkener is interposed between the eye and prism.

The Cross-wires (Fig. 97) are two fibers of spider-web or very fine platinum wire, cemented into cuts on the surface of a metal ring, at right angles to each other, so as to divide the small open space in the center

FIGURE 96..

into quadrants. The intersection of the wires forms a very minute point which, when it is adjusted in the axis of the telescope, enables the observer to fix it upon an object with the greatest precision.

This axis of the telescope (which is also the optical axis of the objective) is usually termed the line of collimation, and the operation of bringing the inter

section of the wires into this axis is called the "adjustment of the line of collimation."

The openings in the telescope tube are made considerably larger than the screws used in adjusting the cross-wires, so that, when the screws are loosened, the whole ring can be turned around for a short distance in either direction. The movable ring at A A (Fig. 96) is used to effect the centering of the eye-piece; and the one at C C is used to effect the adjustment of the objectglass slide.

FIGURE 97.

The Stadia (Fig. 98) is a compound

cross-wire ring, as shown, having three horizontal wires, of which the middle one is cemented to the ring as usual, while the others are fastened to small slides, held apart by a slender brass spring hoop, and actuated by independent screws, by which the distance between the two movable wires can be adjusted to include a given space, as 1 foot on a rod 100 feet distant. These wires will in the same manner include 2 feet on a rod 200 feet distant, or foot at a distance of 50 feet, and so on in the same proportion, thus furnishing a means of measuring distances.

FIGURE 98.

The Supports or Standards of the Transit are firmly attached by their expanded bases to the upper plate, one of

them having near the top a little movable box, actuated by a screw underneath, by which the telescope axis may be raised or lowered and thus made truly horizontal.

ATTACHMENTS OF THE TELESCOPE.—Although the instrument is sometimes used with a plain telescope, oftener it is provided with one or more attachments, as the vertical arc or circle, level, clamp and tangent, and solar attachment.

The Vertical Arc or Circle is firmly secured to the axis of the telescope, and, in the instrument shown, is 6 inches in diameter, divided on silver to degrees and half-degrees, and with its vernier reading to 1 minute. On some instruments the arc is replaced by a full circle.

The Level on the Telescope consists of a brass tube about 6 inches long, each end of which is held between two capstan nuts connected with a screw or stem attached to the under side of the telescope tube. The vial enclosed in the tube is a little over 5 inches long and half an inch in diameter, ground

on its upper interior surface so as to insure an even and sensitive bubble, the length of which is measured by the divided scale above, and thus determines when the bub. ble is brought into the center of its run.

The Clamp and Tan gent consists of an arm, at one end encircling the telescope axis and at the other connected with the tangent or slow

motion screw; the clamp is fastened at will to the axis of the telescope by a clamp-screw inserted at one side of the ring,

FIGURE 99.