474 Lieut.-General Strachey and Mr. Whipple. [Apr. 



of them being carefully made to coincide in direction with the 

 joining the two cameras. 



The charged dark slides, which are separately numbered, so that 

 correction for each of them may be ascertained and recorded, are tl 

 successively placed in the camera and photographs taken of the 

 wires overhead, the pictures of which should coincide with the fiducial 

 lines of the camera, the position of which is as nearly as possible 

 adjusted to secure this coincidence. The photographs thus made 

 preserved, to supply data for correcting the negatives for any error < 

 the fiducial lines, should the slides not be properly adjusted so as 

 secure the coincidence before spoken of. 



Assuming, as may be done without objection for this purpose, 

 the cloud surface photographed and the earth's surface at the pi 

 of observation are in parallel planes, distances measured on the pi 

 graphs from the intersection of the fiducial lines will 

 tangents of angles measured from the zenith to radius equal to 

 height of the cloud. 



Again, if a pair of photographs made simultaneously at 

 extremities of the base are superimposed one on the other, so that 

 forms of the clouds coincide, which they will do accurately if 

 pictures are properly placed, then the line joining the intersections ' 

 the cross lines will represent, both in magnitude and direction, 

 line joining the zeniths of the two ends of the base, from which 

 observations are made, or the base itself. 



If the adjustments before described have been satisfactorily 

 the base, as thus indicated, should obviously fall on one pair of 

 fiducial lines, which, when the photographs are superimposed, she 

 also coincide ; otherwise, if the fiducial lines in the two pictures 

 made to coincide, then the separation of points properly identified 

 the pictures will be the measure of the parallax or angle subt 

 by the base at such points. . 



A scale of angular distance having been prepared as before 

 plained, the parallax thus measured may at once be converted 

 angular measure, and the height of the cloud is given by 

 equation 



H = /3/tan r, 



where v is the angular parallax. 



In like manner, if two photographs taken from the same point u ith 

 an interval of time between them be superimposed, so that the cloud 

 pictures coincide, the line joining the intersections of the cross lines 

 will represent in magnitude and direction the movement or drift of 

 the cloud, and the velocity in miles per hour will be found from the 

 equation 



SB 3600 

 V = -x-* X :r-, 

 p 5280 t" 



