



91.J Cloud Photography. 471 



azimuths and altitudes derived by measurement of a series of points 

 in the clouds, properly identified in the sets of pictures, is very 

 tedious, and a graphical method was suggested by Sir Gr. Stokes, 

 which, though very ingenious, was found to be troublesome in prac- 

 tice, and was not persevered in. 



From the nature of the process employed, the indefinite outlines of 

 the clouds, and their incessant change of form, complicated by the 

 effects of perspective distortion on an irregular and ill-defined surface, 

 it is necessarily impossible to identify cloud-points in the different 

 pictures with much precision or make exact measurements ; and 

 approximate results, therefore, are all that can be sought for. The 

 object of the enquiry is chiefly to determine the velocity of movement 

 of clouds at varying heights above the earth's surface and to obtain 

 the heights of those observed at the greatest elevations, which appear 

 as cirrus. 



If A and JB are the azimuths of any point in a cloud, and 7t a and Z 4 

 the zenith distances, observed respectively at A and B, the ends of 

 the base /3, then the distances, measured in a horizontal plane pass- 

 ing through the base, D tt , D 6 from A and B respectively of the 

 point vertically under the cloud-point will be 



^_ ft'- 



'sin(A-B)' ^sin(A-B)' 



and H, the height of the cloud-point above the horizontal plane passing 

 through l.he base, will be 



H _ * sin (B) sin (A) 



P sin (A B) tan Z a P sin (A B) tanZj' 



hese values are readily found by means of a slide-rule constructed 

 as shown below. The graduations of the upper scale of the fixed rule 

 are log sines ; those of the lower scale of the fixed rule logs of 

 Qumbers, the log of 2400 feet, the length of the base, coinciding with 

 log sin 90. 



The upper sliding rule No. I is graduated with log sines of small 

 ingles on the same scale as the first rule, the point marked with 

 index No. I indicating log sine 5 44' 27", which is 9' 00000, or 



23", which is S'OOOOO. 



e lower sliding rule No. II is graduated with log tangents Z, the 

 joint marked with index No. II, corresponding to log tan 45, and on 

 'he same scale as the sines. 



To apply the rule, bring index No. I of the slide-rule No. I opposite 

 ihe angle A on the upper fixed scale. Then bring the index No. II of 

 ;he slide-rule No. II opposite to the angle A B on the slide-rule 

 ^o. I. 



2 i 2 



