4 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



tripod. Holes should be placed in the blocks to receive the tips of 

 the tripod legs. 



9. Observation platform. — The theodolite platform should, if prac- 

 ticable, be so constructed that no vibration will be transmitted from it 

 to the theodolite. This is accomplished by building a second platform 

 or support for the theodolite, in such a way that there is a space and 

 no immediate connection between the two. Figure 1 shows a section 

 of such a platform and theodolite stand. Three posts in the form of 

 an equilateral triangle, about 36 inches from center to center, are sub- 



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 /ween p/a//brm and 

 /fyeoc/o/Z/e supporf 



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 /or p/a//brm A/ ^ 



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 po/nf 



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Figure l.— Section of observation platform and theodolite stand, showing the insulation of the one 



from the other. 



stantially arranged and rigidly braced. When secured about the 

 observation point, caps for the theodoUte foot blocks are placed over 

 the posts in a common horizontal plane. The observation platform is 

 then constructed with the upper surface of flooring flush with the top 

 of the theodohte foot block but not in any way connected with the 

 theodolite stand. A space of at least half an inch should be left be- 

 tween the two. The observation point will be the common center of 

 this equilateral triangular inclosure. Brass plates, furnished from 

 central office stock, are attached to each of the theodolite foot blocks. 



