34 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



devices may be supplied by dry cells or by the regular house current. 

 In case the latter is used it is, of course, necessary to reduce the voltage 

 by means of a transformer. If the field of view is too bright, the light 

 which is being observed does not show up plainly and, therefore, can- 

 not be followed to such a great distance. This difficulty may be 

 remedied by using a higher-voltage bulb for lighting the cross hairs 

 or by placing, a rheostat in the theodolite-lighting circuit by which 

 the current may be reduced as desired during the observation. 



102. Methods oj computation. — The computation of the horizontal 

 distances of the balloon at the end of each minute may be accomplished 

 by the slide-rule method, graphical method, logarithmic computation, 

 or by reference to tables (table No. 1043 used for ascensional rate of 

 180 meters per minute). In the Weather Bureau either the tables or 

 the slide-rule method are used. The only computations made by 

 logarithms or by natural functions are for checking purposes. 



103. Slide rule used in theodolite observations. The slide rule used 

 by the Weather Bureau for computation of single-theodolite obser- 

 vations is an ordinary polyphase duplex, 10-inch rule. The principle 

 of the slide rule is purely logarithmic, and each scale is graduated 

 according to that principle, but the manipulation of it and the work 

 done with it is purely mechanical and can be readily taken up without 

 the slightest knowledge of logarithms. The scales of this rule used 

 in single- theodolite computation are the tangent scale T, the sine 

 scale S, and their associate scales, D and A, respectively. The tangent 

 scale ranges from approximately 6° to 45° and the sine scale from 

 ^2° to 90°. These scales are graduated into degrees and minutes. 



104. A complete manual of instructions is furnished with each slide 

 rule, and for that reason but little attention need be given here to the 

 manipulation of the slide rule. The supplement at the end of the 

 manual will give much information of practical interest. 



105. Single-theodolite computation. — In single-theodolite computa- 

 tion, usually only the T and D scales will be used, and these in con- 

 junction with the formula — 



tan « = J, will be sufficient. 



e=the observed elevation angle for any one minute, which is 



found on T scale of central slide of rule. 

 h=ihe assumed altitude or height of balloon at end of each 

 minute. It is the product of ascensional rate into time in 

 minutes from release of balloon. This value is found on 

 the lower or D scale of the slide rule. 

 d=the horizontal distance from the observation point to a point 

 directly underneath the balloon. 

 lOG. Slide-rule computation for pilot-balloon work is affected to 

 some extent by the magnitude of the elevation angle, which separates 

 the work into two phases, namely, elevation angles less than 45° and 

 elevation angles greater than 45°. While an explanation of compu- 

 tation involving an elevation angle of more than 45° is given early 

 in section 7 of the Manual, the direct application to pilot-balloon 

 computation can be stated in simpler form, and will follow later. 

 In ordinary computation, the elevation angle is less than 45°, and in 

 such cases the procedure is simple enough. 



