G. S. Cook — Spectrum qf Aqueous Vapor. 266 



Btorm clouds is at timee something very great, I may say 

 enormoua This has long seemed to call for some explanation. 

 I regret to say that I am far from being able as yel to establish 

 any theory, and can only submit en-rain evidence bearing on 

 the question. I conceive thai there arc three causes which may 

 act separately or conjointly to produce the great absorption of 

 light by storm clouds. We may conceive I L) that a storm cor- 

 responds to a great altitude of aqueous vapor, and a consequent 

 great length ot air column traversed by the rays of light. (2) 

 That a cloud allows light to pass by a process of multiple 

 reflection : the particles of mist being so close together, it 

 conceivable that the broken lines representing the actual 

 path of any one ray of light would when rectified have a 

 length of several times the actual thickness of the cloud. (3.) 

 That water when passing from the form of vapor to that of a 

 liquid, i. c, while in the act of condensation, has an abnor- 

 mally great effect. This view has had its advocates, but I 

 know of no physical evidence of such action, and am rather 

 disposed to believe that the idea was put forward merely as a 



sslble explanation of this cloud action, with no other 

 evidence in its favor. A direct test of this third theory of the 

 abnormal effect of saturated air or perhaps more accurately of 

 nascent cloud, has been made repeatedly by the writer. 

 Doubtless, many present have watched from a high mountain 

 the formation of cumulus clouds on a sunny morning, and 

 have noticed that the lower surface is an almost perfect plane. 

 The upper surface may be ever so much broken and piled up 

 in rounded masses and columns, but the lower surface is ever 

 plane. As the air rises steadily, the upper part of the cloud is 

 seen to ascend while the lower part is stationary, the cloud 

 particles here being renewed as fast as they rise. At the lower 

 surface, then, we have continuously nascent cloud, and light 

 traversing this stratum should show abnormal absorption 

 if it exists. The result of observation is negative as far 

 as recognizing such an effect is concerned. It must be said, 

 however, that the illumination is poor necessarily, and for this 

 reason it cannot be claimed that the theory is disproven. 



Xow while more than one of the above causes of absorption 

 may well be concerned, it seems that any very marked pre- 

 dominance of one of them can be discovered if it exists. For 

 this reason, I was interested to determine if when both stations 

 were immersed in a storm cloud the 3,000 feet of cloud be- 

 tween would give much difference of line. But storm clouds 

 will not come at one's bidding ; and when they do come they 

 are very likely to come at night, consequently the amount of 

 evidence is not very satisfactory. Only twice were the con- 

 ditions favorable, that is, on only two days could the tests be 



