48 



14. The levels of the upper plate are perpendicular to its axis. 



15. The vertical axis of the upper plate coincides with the axis of the lower plate. 



16. The lower plate is perpendicular to its axis. 



17. The center of the vernier plate lies in the axis of the lower plate. 



18. The axis of the plumb-bob coincides with the vertical axis of the instrument. 

 Of the above points the maker attends to numbers 1, 2, 4,5, 6, 8, 9, 11, 12, 13, 15, 



16, 17, 18, as a part of the skillful manufacture of the instrument; and the engineer 

 has no facilities for adjusting them, away from the shop. Points numbered 3, 7, 10 

 and 14 are attended to by the maker when the instrument leaves the shop ; but owing 

 to their liability to derangement, from accidental rough usage, the maker leaves ft 

 in the hands of the engineer to restore them at any tune. It is to these adjustments 

 only that the following remarks are confined. 



Adjusting. 



If the instrument is out of adjustment generally, the engineer will find it profit- 

 able to follow the makers in not completing each single adjustment at once, but 

 rather bring the whole instrument to a nice adjustment by repeating the whole 

 series. 



After setting up, bring the two small levels each parallel to a line joining two 

 of the opposing leveling screws. Bring both bubbles to the center of the level tubes, 

 by means of the leveling screws. In doing this, place the two thumbs on the inner 

 edges of the two leveling screws, parallel to the bubbles, and the fore fingers of 

 each hand on the outer edge. Turn the leveling screws so that both thumbs move 

 Inwards or both outwards. In the former case the bubble will move toward the 

 right, in the latter case toward the left. 



Now turn the instrument 180 in azimuth. If the small levels still have their 

 bubbles in the center of their tubes, these levels are adjusted, and the circles are 

 respectively as nearly horizontal and vertical as the maker intended them to be. 



If the bubbles, however, are not in the center of their tubes, then bring them 

 half way back by means of the leveling screws, and the remaining half by means 

 of the adjusting screw at the end of each of the level tubes. 



It may be necessary to repeat this adjustment several times, but when made, the 

 instrument once leveled will have its small levels in the center of their tubes through 

 au entire rotation of the circle. 



There is one adjustment common to all telescopes used in surveying instruments, 

 that of bringing the cross hairs to a sharp focus, at the same time with the object 

 under examination, the adjustment for Parallax. 



Point the telescope to the sky, and turn the eye-piece until the cross hairs are 

 sharp and distinct. Since the eye itself may have slightly accommodated itself to 

 to the eye-piece, test the adjustment by looking with the unaided eye at some dis- 

 tant point, and while still looking, bring the eye-piece of the telescope before the 

 eye. If the cross hairs are sharp at the first glance, the adjustment is made. Now 

 focus in the usual manner upon any object, bringing the cross hairs and image to a 

 sharp focus by the rack-work alone. A point should remain bi-sected when the eye 

 is moved from one side of the eye-piece to the other. 



To make the vertical cross-line perpendicular to the plane of the horizontal axis, 

 Fiin ply bi-sect some point in the center of the field of view of the telescope, and 

 aote whether it continues bi-sected by this cross-line throughout its entire length 

 when the telescope is moved in altitude. If it docs not. and the point is to the right 

 ol til-- line in the upper part of the field, the adjustment is made by loosening the 

 four capstan-headed screws, and rotating the reticule in the direction of a lelt-handed 

 screw, until the cross-line is moved over half the distance between the point and 

 the line. Again, bi-sect the point by means of one of the tangent screws. It should 

 now remain bisected throughout the length of the cross-line. 



To adjust the horizontal wire and the long bubble so that they are parallel, we may 



proceed as follows : Select two bench marks which are at approximately the same 



tion. I'luce the transit near one of them so that when a rod is held on this point 



I ie teli -scope will be about a quarter of an inch from the rod. The height. 



of thr Instrument al>o\e the bench mark may be determined very accurately l>y li>i>kin<j 



""" 'Live at the rod and setting the target or a pencil point in the o 



of th- \..w turn the telescope to sight the other bench mark and take a rod 



leading on it in the n-nal way. being careful to place the bubble in the center <>l the 



e of the two rod readings is the difference in elevation between the 



- plus or minus the error of the line of sight. Now place 'he transit at 



