INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 59 



183. The angle 6 in the foregoing discussion is measured by a grati- 

 cule, which gives the apparent length of the tail as seen in the tele- 

 scope. The unit of the graticule is equal to some fraction Ijk 

 of a radian, the radian being the unit angle in a circular measure. 

 Hence, if the apparent length of the tail is w graticule units, the angle 

 is equal to m {Ijk) radians and hence 



CB= — cos E sin. E and 0C= — cos E cos E. 



m m 



184. The graticule. — The graticule consists of a glass scale placed 

 in the focus of the telescope or of an auxiliary horizontal cross hair 

 operated by means of a micrometer drum. Thus, the tail of the bal- 

 loon is seen moving among the lines of the graticule or is brought 

 between the fixed and movable cross hairs, and its apparent length 

 can be estimated. 



185. To obtain the value of the graticule scale {k) of a theodolite, 

 the following procedure should be adopted: 



Set up a foot rule vertically at a measured horizontal distance of 

 100 feet from the object glass of the theodolite. The center of the 

 rule should be in the same horizontal plane as the eyepiece of the 

 theodolite. Focus carefully and determine as accurately as possible 

 the number of graticule divisions {n) corresponding with the foot 

 length, as seen in the telescope. Since the angle subtended by the 

 rule is 0.01 radian, the value of k will be 100 Xw. It should be noticed 

 that k may differ for different theodolites. 



186. Length and form of tail. — It is advisable to adopt a standard 

 length of tail for each theodolite so that the value of "kl," used in 

 the computations, will always be uniform. The standard length 

 should be between 25 and 35 meters. This, however, will usually 

 be too long to appear in the field of view of the telescope during the 

 first few minutes of the ascension. A subsidiary pendant should then 

 be used until the full length comes within the field of view. This 

 subsidiary pendant should be placed at a quarter or a fifth of the 

 total distance of the full tail from the balloon, to simplify the mental 

 calculations. 



187. The tail consists of a double length of ordinary thread or 

 silk. The end of the tail is marked by a pendant consisting of a 

 foolscap sheet of very thin paper having a strip of very fine aluminum 

 wire gummed along one of the shorter edges, while the opposite edge 

 of the paper is folded over and gummed to assist in keeping the 

 paper fairly rigid. The end of the thread is tied around the aluminum 

 wire through a smaU hole in the paper. The visibility of the pendant 

 is often improved by the use of colored paper, selected according to 

 the appearance of the sky. Silvered paper is rather better than white 

 paper with a clear, blue sky ; red is suitable for a hazy or cloudy sky ; 

 and black, for an overcast sky. 



A form of tail which spins is to be avoided, because it causes the 

 thread to shorten and gives values of heights and velocities which 

 are all too high. 



188. In determining the free lift of the balloon the weight of the 

 tail is to be corrected for in exactly the same manner as is the weight 

 of a lantern or electric-lighting unit. 



