38 



The graduations on the compass box should begin at the North point, and run 

 90 iu both directions ; then decrease to again at the South point. In order that 

 the needle reading may indicate the direction of the telescope, the line joining the 

 zeros of the ordinary compass ring must be in the same vertical plane, with the 

 line of colliraation of the telescope ; and the letters denoting the cardinal points, 

 East and West, must be transposed; i. e., when the letter N is towards the North, 

 the letter W should be towards the East. Of course the needle indicates magnetic 

 north, and in the case of instruments unprovided with means of setting off the local 

 variation of the needle, all the readings of the needle must be corrected for this 

 local deviation. 



Spirit-Levels . 



The spirit levels, as regards their sensitiveness, should be in strict keeping with 

 the optical power, and the graduations of the instrument, but the quality should be 

 of the best. A level-bubble should move uniformly over the same distance, when 

 the telescope is made to point on two objects alternately, differing slightly in alti- 

 tude, by the leveling screws alone. In change of temperature the bubble should 

 lengthen symmetrically from the center ; and no matter what its length, it should 

 move quickly, without any of the hitching, which is caused usually by a little d.'rt 

 introduced when it is filled. 



Of the three levels attached to the complete transit, the telescope level is the 

 most sensitive. It should be sensitive enough for ordinary leveling, such as good, 

 railroad work. The level in front, or at right angles to the standards, should be 

 sensitive enough to make a line plumb by it to any height ; while the third lev 1 

 on the standard is used in leveling up the instrument, and to establish the z^r 

 point for the vernier correctly when vertical angles must be measured. 



The test of the fitness of the various levels for the capacity of the instrument 

 should lie in this : that after carefully bi-secting an object in the field of view, in 

 such a position of the instrument that all the levels can be read, and then slightly 

 deranging them all with the leveling screws, the bi-section will be accurately made 

 after restoring the levels to the exact position they before occupied, by the leveling 

 screws alone. 



Leveling Screws. 



Messrs. C. L. Berger & Sons usually cut their leveling screws with 32 threads to an 

 inch provide the usual four screws in opposing pairs. The plates once set firmly 

 apart by tightening two of these screws on the same side, the leveling of the instru- 

 ment is easily accomplished by turning the two screws of an opposing pair so that 

 both thumbs shall move toward each other (when the bubble will go toward the 

 right), or both thumbs away from each other, when the bubble will move toward 

 the left. Instruments intended for triangulation, i. e., reading to 10" or less, should 

 however be supported on three, instead of upon four screws. In this case the in- 

 strument is rapidly leveled by bringing one level parallel to two of the screws, the 

 other level will now be at right angles to it. Level both levels at the same time 

 by turning one of the screws to which the first level is parallel and the screw 

 which is at right angles to this level. Of course the instrument may now be 

 reversed to guard against non-adjustment of the levels. 



Three Leveling Screws versus Four. 



To the student of the progress in Engineers' field instruments, the question often 

 presents itself as to the comparative merits of an instrument provided with three, 

 over one having four leveling screws. It should be here rem.-irUed that the greater 

 portability existing in instruments provided with four leveling screws still com- 

 mends it It to all using the more customary class of instruments. However, the 

 finest class of field instruments, requiring s'pirit-levels corresponding to the fine- 

 ness of graduation, cannot be advantageously manipulated with four leveling 

 icrews. The results thus obtained would be little better than those obtained with 

 a more ordin.-iry inst rument. To insure the full benefit of a finer instrument, such 

 as used in trlangulation, the maker will prudently apply three leveling screws, 

 mounted on a basis larger then is usual in instruments with four screws. So, while 

 four leveling screws have the advantage of greater compactness and less weight 

 iave the advantage for closer setting, giving better results. The 

 maker will therefore adapt either the one or the other kind to his instruments as 

 the case may require. 



