1891.] 



Cloud Photography. 



47D 



1 mile, and divide it into 60 equal parts, and from the zero point on 

 the horizontal line draw radiating lines through the points of division, 

 extending to the vertical at 120 seconds. This constitutes a scale of 

 proportional velocities from to 60 miles per hour, and may be ex- 

 tended to any higher velocity. N"ext (see fig. 4) draw two parallel 



vertical lines at a distance apart equal to the length of the base, 

 2400 feet, on the scale before assumed to represent 1 mile, and draw 

 a horizontal line intersecting the other two at right angles at points 

 M and N. 



Then mark off the length of drift t upwards on each of the two 



