54 A MANUAL OF TOPOGRAPHIC METHODS. 



surface of the glass a reflection of the sun, producing an imaginary sun 

 The mirror should not be moved until this imaginary sun, moving with it, 

 appears to rest on the object to which the flash is to be cast, as the hill on 

 which the triangulator is standing. As both surfaces of the mirror are par- 

 allel, the true reflected rays of the sun from the surface of the mirror will 

 also be cast on the object sighted to. 



This instrument is in great favor, especially with the Western parties, 

 where portability is a matter of moment, first, because it is light and con- 

 venient to carry and use, and second, because there are no movable parts 

 to get out of adjustment by jarring. This latter is a serious defect in the 

 Coast Survey instrument, since unless frequently tested the two rings may 

 have moved, thus causing the reflection to be cast out of parallelism with 

 the hue of sight of the telescope. 



The use of heliotropes presupposes the employment of men to operate 

 them, thus increasing materially the expense of the work. Misunderstand- 

 ings continually arise between the heliotropers and the observer, causing 

 vexatious delays, and therefore their employment should be avoided when- 

 ever possible. 



THEODOLITES FOR TRIANGULATION. 



Several instruments differing widely in power and degree of accuracy 

 have been in use for the measm-ement of angles in the primary triangula- 

 tion. Formerly theodolites having circles 6, 7, 8, 10, and 11 inches in 

 diameter and reading by vernier to 10 seconds were employed, and the 

 results were reduced and adjusted by Least Squares. Subsequently, it 

 appeared desirable to employ a higher class of instruments and thus obtain 

 more accurate results, which would render unnecessary this tedious adjust- 

 ment. Pursuant to this decision the use of these vernier theodolites has 

 been, in the main, discontinued, and theodolites having 8-inch circles, read- 

 ing by micrometer microscopes, have been substituted almost universally 

 in the primary w< irk. 



One of these theodolites is represented in PI. v and Fig. 7. 



The circle, as was above stated, has a diameter of 8 inches, and is sub- 

 divided to 10 minutes. The object glass is 2 inches in diameter and its 



