of several hundred feet and, having made the previous adjustments, clamp the lower horizontal motion and revolve the telescope completely over, so as to point in the other direction. Place there some well-defined object, as a tack in the end of a stake, and at practically the same distance from the instrument as the first object selected. Unclamp the upper. plate and turn the instrument half-way round or through. 180 degrees, as indicated by the vernier, and direct the telescope to the first object sighted, B (Fig. 66). Again bisect

this with the wires, clamp the instrument, and revolve the telescope over and observe if the vertical wire bisects the second object, C, when the telescope is now pointed at it from the reverse position. If it does, the line of collimation is in adjustment. If not, the second point observed, as E, will be double the deviation of that point from the true straight line, as the error is the result of two observations made when the wires were not in the optical axis of the telescope. In the last pointing of the instrument, after the telescope was directed the second time to B, the point bisected at E was situated as far to one side of the true straight line, BAC, as was the point first sighted, D, on the other side. To correct the error, use the capstan-head screws on the side of the telescope and move the vertical wire to one side or the other by one-fourth the distance, keeping in mind the fact that the eyepiece inverts the position of the wires, and that in moving these screws the observer must operate them as if to increase the error noted. Unclamping the instrument and swinging it around so as once more to bisect B, again revolve the telescope, and if the adjustment has been correctly made

the wires will now bisect the central point, C. Test the adjustment by revolving the instrument half-way round again, fixing the telescope on B, clamping the spindle, and once more revolving the telescope on C, and repeat the observations and adjustment of the wires until the correction of the collimation is completed.

The adjustment of the standards is the next and last important adjustment of the transit, and this is made in order. that the point of intersection of the wires shall trace a vertical line as the telescope is moved up and down. This result is only obtained when the two standards which support the axis of the telescope are at the same height. Point the telescope to some object which will give a long vertical range, as at a star and its reflection in a bath of mercury, or the top of a tall church spire and the center of its base, or a long plumb-line. Fix the wires on the top of the object and clamp the spindle, then bring the telescope down until the wires bisect some good, well-defined point at the base. Turn the instrument half-way round or through 180 degrees, revolve the telescope, and focus the wires in the lower point. Clamp the spindle and raise the telescope again to the highest point. If the cross-hairs again bisect it, the adjustment is perfect; if they pass to one side, the standard opposite to that side is highest, the apparent error being double. This is corrected by turning a screw underneath one of the axes which is made movable, the correction being made for half of the amount of the apparent error.

87. Traversing with Transit.-A traverse line executed with the transit differs from one executed with the planetable or the theodolite because of the ability to transit the telescope or revolve it through 180 degrees vertically. As a result of this construction of the instrument the angle between backsight and foresight which is read and recorded is not the full horizontal angle observed by swinging the instrument in azimuth, but it is the deflection of the new

direction, or of the foresight, to the right or left of the backsight prolonged.

Having set up the instrument at A (Fig. 67), direct the telescope at the first point in the traverse B, with the graduated circle set at zero and by using the lower motion.

zero.

FIG. 67.-TRAVERSING WITH TRANSIT.

Record the angle zero and the distance AB measured by chain, stadia, or other method. Move to B, and setting up and plumbing the instrument at that point, backsight on the point A, using the lower motion and with the circle still at Clamp the lower motion and transit the telescope. The instrument will now point in the direction of A', which is the prolongation of AB if the collimation be in perfect adjustment. Loosen the upper clamp and point at the new foresight C, and then reclamp the vernier. The angle a is the deflection from the straight line AA' to the right towards C. In like manner the instrument is moved to C, and the line BC prolonged to B' by transiting the telescope, and the angle ß, from B' to D, is recorded as a left deflection.

EXAMPLE OF TRANSIT NOTES.

The notes of such a transit traverse are kept in the following manner: In the first or Station" column is recorded

the total distance in hundredths plus single feet from the initial point. In the second or "Distance" column is recorded the distance between two stations A and B. In the third column is recorded the deflection angle with plus or minus signs, according as the deflection is to right or left. In running a simple traverse nothing further is requisite than the above. If, however, as is likely to be the case in topographic surveying, it is desired to know also the bearing of the line, the true azimuth of some sight, preferably the first. line, should be determined by observation on Polaris (Art. 312), and the magnetic declination (Art. 92) should be noted, as well as the true or transit declination, by reading the angle between the azimuth line and the first line of the traverse. This angle should be recorded in the fourth column, "True Bearing." Then, as the traverse is run, the deflection right or left should be added to or subtracted from the last true bearing and thus give the new true bearing. For a check the compass-needle should also be read and recorded in the column "Magnetic Bearing," and the true bearing should agree with this approximately by the difference of the declination observed. There should be a last column in which to record remarks of streams passed, road junctions,

etc.

On the opposite page of the note-book, facing the notes, there should be ruled a vertical line through the center of the page, and the customary process of recording the objects encountered on the traverse line is to use this vertical line as the line of the traverse. Beginning at the bottom of the page, plat the first station, A; then, at the proper distance above A, plat, still on the center of the line and disregarding the deflections, the second station, B. Crossing this line of the traverse, note the topographic features, as streams, roads, houses, etc. Where topographic notes are taken in detail it is practically impossible to keep a proper record of the traverse by considering it as a straight line; in which case,

instead of using a central line as the traverse line, an irregular line should be drawn up the page, each tangent or deflection line of the traverse making an angle with

Town

FIG. 68.

VERSE.

71+

Creek

59 +

55

the last, which shall agree as nearly as 78+ possible, by eye estimation or by platting with horn protractor, with the angle made on the ground. By this means the topography of the country will not be distorted in recording it on the sketch page (Fig. 68).

PLAT OF

88. Platting Transit Notes with Protractor and Scale.-Transit notes. may be platted in two ways:

1. By means of a protractor and scale, and

2. By latitudes and departures (Art. 90).

In platting with a protractor (Art.

TRANSIT ROAD TRA- 89) and scale, set the center of the protractor over the occupied station as platted on the map, set the zero on the prolongation of the last sight, and plat off the deflection to right or left by the proper number of degrees. Then, removing the protractor, plat on this new deflection line the proper distance to scale. This deflection line should be drawn sufficiently long, so that when the protractor is centered over the second station this old deflection line will appear on the map as the zero-point on which to set the protractor for the next following deflection.

89. Protractors.-In the platting of traverses run with a prismatic compass, the simplest form of a semicircular horn protractor will fill the requirements; also in platting reconnaissance triangulation in order to determine the relative positions of stations. Where any attempt is made at accurate platting, as of traverse run with transit, a full-circle vernier arm protractor should be used (Fig. 69). Where