78 



DISCOVERY 



mark records the proper direction of the object. The 

 scale is then clamped into position and the instrument 

 is ready to be used for observation. 



The second operation is to inflate the balloon. 

 The balloon is weighed and, from the Dines formula, 

 the weight it should just lift, so that the rate of ascent 

 may be 500 feet per minute, is calculated. The balloon 

 is accordinglj- filled with pure hydrogen from a cylinder 

 of the compressed gas until it just lifts the calculated 

 weight. The orifice in the balloon is sealed and all 

 is ready for the flight. 



Generally two observ-ers work together in taking 

 an obserV'ation,and,all being ready, one of themreleases 

 the balloon and at the same time starts a stop watch. 

 The other sights the balloon in the telescope of the 

 theodolite and follows its motion by moving the two 

 tangent screws in appropriate directions. Exactly 

 at the end of successive minutes the first observer reads 

 the scales of the instrument, and the observation is 

 continued until the balloon is lost to sight. This may 

 occur from many causes. The balloon may enter 

 cloud or haze, or it may burst or be lost in distance. 

 Should the observation terminate by the balloon 

 entering cloud, the height of the cloud becomes very 

 accurately known. 



As a result of the observation, the position of the 

 balloon at the end of each minute is known. From 

 these figures it is easy to compute the speed and direc- 

 tion of the wind in each minute and, further, as each 

 successive minute represents an additional 500 feet in 

 altitude, the heights at which the various winds occur 

 are also known. In this way upper winds can be 

 measured up to as great a height as a pilot balloon can 

 be followed through the theodolite. On a clear day, 

 if the wind be not too strong, heights of 40,000 feet are 

 by no means impossible. The necessary calculations 

 can be effected most readily on a special slide rule 

 made by the Air Ministry, and when the balloon is 

 finally lost, the whole of the results are instantly 

 available for use. 



A More Accurate Method for Special Uses 



It will be at once appreciated that the accuracy 

 of the method is primarily dependent on the uniformity 

 of the rate of ascent of the pilot balloon. Should this 

 depart widely from the theoretical value of 500 feet 

 per minute, the results obtained will be vitiated. On 

 most daj-s the movement of the atmosphere in a vertical 

 direction is small and the error introduced by assuming 

 a steady rise of the balloon is negligible from a practical 

 point of view. For certain investigations, however, 

 it is necessary that uncertainties due to variations in 

 the rate of ascent should be avoided. Thus in experi- 

 mental gunnery it is important that absolutely accurate 

 values of upper winds should be available. So also 



in investigating the structure of the atmosphere at 

 great heights ; here the balloon has expanded so much 

 owing to the decreased external pressure that porosity 

 or pin-holes develop and the rate of ascent ceases to be 

 500 feet per minute. Cases are by no means rare in 

 which the baUoon starts to fall, so that calculations 

 based on a uniform rate of ascent will be grossly mis- 

 leading. To obviate the difficulty, the balloon is 

 followed by two theodolites, one at each end of a 

 measured base line. 



The sets of figures thus obtained at the intervals of a 

 minute enable the position of the balloon in space to 

 be fixed, and no assumption as to the rate of ascent has 

 to be made. The method, though of greater accuracy 

 than when only a single theodolite is used, nevertheless 

 demands more personnel, more equipment, and good 

 telephone communication. The amount of computing 

 is also much greater, and the results are not therefore 

 available so quicldy. For these reasons this method 

 is not in general use. 



A War-time Method 



If the speed and direction of the wind at, say, 20,000 

 feet are required, it will be noted that it would take a 

 pilot balloon 40 minutes to attain this height, pro- 

 vided it did not burst in the meantime. Occasionally, 

 especially during the war, such a wind value was re- 

 quired very quickly , and an ingenious method of effecting 

 this was invented. It consisted in firing a round of 

 anti-aircraft shell with fuse set to burst the shell at 

 the desired height. A puff of smoke was thus pro- 

 duced which was carried along by the wind prevailing 

 at that height. The reflection of this smoke-burst was 

 observed in a horizontal mirror, the observer looking 

 through a fixed peep-sight at a known height above 

 the mirror. The procedure was to make a dot on the 

 mirror where the reflection appeared to be, and then 

 to make a simUar dot, say sixty seconds later, in the 

 new position of the burst. The line joining these two 

 dots was thus the path of the reflection as seen in the 

 mirror. Simple mathematics obtain the result that the 

 length between the dots bears the same proportion 

 to the height of the peep-sight above the mirror as the 

 actual distance covered by the smoke burst bears to 

 the height of the burst. As three out of the four 

 terms of this proportion are measurable, the unknown 

 term, namely, the distance travelled by the smoke, 

 becomes immediately calculable. As this distance was 

 covered in sixty seconds, the speed of the wind follows 

 very simply. The wind direction is readily obtained 

 by suitably orienting the mirror. 



Should the day be persistently overcast with low 

 cloud, all the foregoing methods are of very little use. 

 Recourse must then be had to very special methods 

 which it is not proposed to deal with in the present 



