INSTRUCTIONS FOR MAKING PILOT BAULOON OBSERVATIONS 7 



heavy ring or handle, H, is supported above the tripod head, T, by 

 means of three levehng screws, LS. Each levehng screw is provided 

 with a tension or clamp screw, L. Pendent from the vertical axis 

 and center of instrument VA is a small chain and hook, p, for the 

 attachment of a plumb bob or anchor rod. 



14. The vernier. — Essentially, the vernier, figure 3, hv, consists of 

 a small graduated scale the unit divisions of which are just a certain 

 amount smaller than the divisions of the scale upon which it is applied. 

 This is accomplished on the circles of the theodolite by taking a space 

 equal to 9°, laying it off on the vernier, and dividing it into 10 equal 

 parts. In figure 3, drawing A, let HC be the horizontal circle 

 divided into degrees, and hv the horizontal vernier. Note that while 

 the zero of both HC and hv are coincident, the tenth division of hv 

 is coincident with only the ninth division on HC. Thus, each division 



HC 



1 



o 

 3(>o 





 360 



Figure 3.— Theodolite vernier. 



of hv is one-tenth degree less than each division on HC. When two 

 such scales are placed together, some particular line of the one will 

 always be coincident, or nearly coincident, with one of the divisions 

 on the other. The position of the coinciding divisions, or the nearly 

 coinciding divisions, determines the vernier reading. For example, 

 when the third, fourth, or sixth division of hv is coincident with some 

 division on HC, the fractional parts of degree will be 0.3, 0.4, oi 0.6, 

 respectively. On drawing B of figure 3 the vernier reading is 0.6 of 

 a degree, and on drawing C it will be noticed that no one division of 

 hv is coincident with any other of the scale HC but that the seventh 

 and eighth of hv are both between two of the divisions of HC, which 

 shows that the vernier reading is more than 0.7 and less than 0.8 of 

 a degree. The second place of the vernier reading must be gained 

 by estimating the fractional part of one of the vernier divisions, 

 which is represented by the space between 6 on HC, and 7 on hv. In 



