Cloud Photography. 4(57 



April 30, 1891. 

 Sir WILLIAM THOMSON, D.C.L., LL.D., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The following Papers were read : 



"Cloud Photography conducted under the Meteorological 

 Council at the Kew Observatory." By Lieut.-General R. 

 STRACHEY, R.E., F.R.S., and G-. M. WHIPPLE, Superintendent 

 of the Observatory. Received April 23, 1891. 



In 1878 the Meteorological Council decided upon undertaking a 



iries of experiments with the view of attempting by means of photo- 

 graphy to obtain a record of the height and velocity of the clouds, as 

 indicating the movements of the upper parts of the atmosphere. For 

 this purpose a plain cubical camera was constructed, with its optical 

 axis directed to the zenith, and a number of pictures of clouds were 

 thus obtained. The results were so far satisfactory as to establish 

 the possibility of identifying points in the clouds which would admit 

 of the calculation of their height with considerable precision. But, 

 owing to the small field of view of the lens made use of, it was found 

 that the opportunities of photographing clouds in this manner were of 

 somewhat rare occurrence, and it was therefore decided, on the pro- 

 posal of Captain Abney, to whom the Meteorological Council is 

 indebted for his valuable advice throughout the course of these 

 experiments, to construct two cameras so arranged as to enable them 

 to be directed to any part of the sky, and thus to photograph clouds 

 all positions. 



For this purpose the cameras were fitted with theodolite mountings, 

 provided with altitude and azimuth circles. The dark slides for 

 carrying the sensitised plates were fitted with glass plates, upon 



hich cross lines indicating the position of the optical axis were 

 ;hed. These lines were photographed simultaneously with the 

 louds, and the readings of the divided circles, recorded at the time of 



posure, thus supplied the altitude and azimuth of the point of the 



oud covered by the intersection of the cross lines at that moment. 



From a photographic picture of a series of staves erected at known 

 ingular intervals, a scale of angular distances was obtained, by means 



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