8 



INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



drawing C of figure 3, this space is about half of one-tenth, or 0.05 

 of 1 degree. Thus we see the vernier reading in this particular case 

 is 0.75 of a degree, which, added to the index reading of the scale, 

 determines the degrees and hundredths. The practical application 

 of these verniers is shown by the sectional view of the theodolite in 

 figure 4. The accompanying table gives the reading of each vernier 

 in figures 3 and 4. 



vv 



Figures A. 

 Figure 3 B 

 Figure 3 C. 

 Figure 4... 



I IV 



0.00 



.60 



359. 75 



315.00 



Figure 4. — Section of theodolite sliowini: arranireinent of verniers with horizontal and vertical circles 

 (/7V, horizontal limb and front vernier; hv, horizontal limb and ri?ht or 45° vernier; VC, vertical circle; 

 VV, vertical limb and vernier). 



15. Micrometers. — Some of the theodolites now" in use are equipped 

 with micrometer drums instead of verniers for the one-tenth degree 

 readings. The drums are graduated to tenths of a degree and the 

 graduations numbered from to 9. In reading such instruments 

 the nearest whole degrees are read from the circles and the tenths 

 directly from the micrometer drums. 



IG. Assembling the theodolite. — Assuming that the crates have been 

 removed and that no damage has been done either in shipping or 

 unpacking, the tripod will be opened up and planted firmlv upon 

 the floor with legs well spread and securely set to prevent slipping. 

 Loosen the milled tension nut of the shifting center, figure 2, and run 

 well down to the knurled head of spindle. Then remove the round 

 W'ooden cap. Loosen the shifting-center nut or clamp ring, CR, fig- 



