718 Mr. C. J. P. Cave [April 11, 



to minute can be determined, and from its trajectory, or its path, as 

 it ascends, the winds that it encounters can be calculated. 



The theodolite used is constructed specially for the purpose ; a 

 prism in the telescope reflects the light at right angles, so that the 

 observer is always looking in a horizontal direction, even if the 

 balloon is overhead. It is important that the observer should be in 

 as comfortable a position as possible, for an ascent sometimes lasts 

 over an hour and a half, during which time the observer can only 

 take his eye from the telescope for a few seconds at a time, otherwise 

 he may lose sight of the balloon and be unable to find it again. 



The balloon having been started from one end of the base, ob- 

 servations are taken from both ends at exactly the same times, usually 

 every minute. From the positions of the balloon at each successive 

 minute, which are plotted on a diagram, the run of the balloon during 

 the minute can be measured, and hence, the wind velocity during 

 that minute can be obtained. After the wind velocities have been 

 measured off, and the wind directions obtained from the directions of 

 the lines on the diagram, another diagram is constructed showing 

 the relation of the wind velocity and direction to the height. 



It is not necessary, however, to have two observers if the rate of 

 ascent of the balloon is known ; in such a case, the complete path of 

 the balloon can be calculated from the observations of one theodolite. 

 It is not, however, possible to know the rate of ascent with complete 

 accuracy, as up and down currents in the air will affect the normal 

 rate. In practice, especially in clear weather, the method is fairly 

 satisfactory. The method of one theodolite requires less preparation, 

 and the subsequent calculations of the path of the balloon are less 

 laborious, than in the case of observations taken with two theodolites 

 from opposite ends of a base line. 



The beet, time for observations is towards sunset, so that the 

 balloon reaches its greatest height after the sun has set on the surface 

 of the earth : at such times the balloon, still illuminated by the sun, 

 shines like a planet, and on one occasion I should have found it im- 

 possible to tell which was the balloon and which was '\'enus, except 

 for the movement of the balloon. The distances at which balloons 

 may be seen through the telescope of the theodolite are remarkable. 

 A striking instance was when the flash of the sun on the small 

 meteorograph, was seen, not once, but repeatedly, when the balloon 

 was about nine miles above the sea and at a horizontal distance of 

 about thirty miles. 



In considering the structure of the atmosphere, as it has been 

 revealed by the observations I have carried out, principally at Ditcham 

 on the South Downs, we may divide the subject into two parts : 

 firstly, the wind structure in the lowest kilometre, and secondly, the 

 general wind distribution up to the greatest heights reached by the 

 balloons. 



It is a matter of common observation that the wind increases 



