OF ARTS AND SCIENCES. 203 



XX. 



HEIGHT AND VELOCITY OF CLOUDS. 



By Professor E. C. Pickering. 

 Presouted, Jan. 11, 187G. 



The velocity of the wind at different altitudes is an important 

 element in Meteorology, and the ordinary methods of measuring it are 

 far from satisfactory. By the following method, it is believed that the 

 velocity of the wind at considerable heights may be measured with an 

 accuracy at least equal, and probably greater, than that of similar 

 measurements near the surface of the earth. The apparatus consists 

 simply of two similar camera obscuras formed of tripods covered with 

 black cloth, and with cosmorama lenses above, which form an image of 

 objects near the zenith on a sheet of paper placed beneath. A day is 

 selected when cumuli clouds are crossing the sky, and the two cameras 

 are placed at any convenient interval, as a hundred metres, in a direc- 

 tion nearly perpendicular to the direction of the wind. An observer 

 with a watch is stationed at each camera, and when a cloud enters the 

 field a signal is given, and each draws a line tangent to the edge of the 

 cloud and parallel to the direction of the wind every half minute. At 

 the intermediate quarter minutes, other lines are drawn perpendicular 

 to these, and also tangent to the cloud. The first series of lines will 

 be nearly coincident, the second at intervals marking the cloud's 

 motion. The zenith is now marked on each drawing by suspending a 

 plumb-line from the centre of each lens, or in some other way, and a 

 line drawn through it parallel to the direction of the cloud's motion. 

 It will now be found that the distance of this line from those parallel 

 to it and tangent to the cloud is different in the two sheets by an 

 amount equal to the parallax of the cloud, or the angle between the 

 two cameras as seen from the cloud. The height of the cloud may 

 then be easily determined, if we know the focal distances of the lenses 

 and the interval between the cameras, by the jiroportion : Difference 

 of distances of the two lines : focal length of lenses = interval between 

 the cameras : required height of cloud. To determine the accuracy of 

 this method, suppose the interval between the cameras one hundred 



