1891.] 



Cloud Photography. 



477 



id a second value of the cloud height is obtained, which serves to 

 mfirm or modify the first determination. 



Then pictures A! and A 2 are placed in the frame, and the images 



Derimposed and made to coincide as before, but now the distance 

 sparating the zenith of the two pictures, which will be termed the 



ift, will indicate the space the cloud has moved during the interval - 

 jetween the taking of the two pictures ; and the angle which the line 

 joining the zeniths makes with the line of base gives the direction in 



lich the drift has taken place. 



From the length of the drift measured upon the plates as above, the 

 jlocity of motion may easily be obtained by a graphical method. 



