268 C. 8. Cook — Spectrum of Aqueous Vapor. 



appeared to be the same, but the aqueous vapor spectrum was 

 very faint. To quote from the record made at the time : "At 

 noon with clouds of the same darkness, and with the circum- 

 stances all apparently the same as on yesterday, the line is very 

 faint. Hence the clouds at noon, at the time of their greatest 

 illumination, gave readings of as" great difficulty as yesterday, 

 and yet they are all persistently 0." On the preceding day,, 

 they had averaged 21. At four in the afternoon the cloud 

 grew evidently thin, and at six the sun could be easily dis- 

 tinguished. This is but one of many instances that might be 

 quoted. An excellent example of the discriminating power of 

 the instrument in this direction is worth giving here. During 

 daylight of Aug. 5, 1886, a storm rapidly gathered. During 

 the night rain fell steadily. During the forenoon of the next 

 day, Aug. 6, it rained very heavily. We were closely wrapped 

 in cloud all day, the vapor line being 15 to 30. The third 

 morning we found ourselves in cloud as usual. The spectro 

 scope gave a vapor line of 0. At the breakfast table, at about 

 nine, one of the guests (a professor of Civil Engineering, who 

 ought to be a good practical weather prophet if anyone, surely) 

 remarked it was a most discouraging outlook. I was very 

 happy to assure him that " the cloud was only a cap, or at most 

 part of a thin sheet of cumulus cloud which could not possibly 

 give us any more rain ; that the storm was all past, and nothing 

 but fair weather clouds remained." In about two hours the 

 clouds lifted, revealing the clearest of atmospheres, and show- 

 ing us that the cloud layer was very thin broken cumulus. It 

 was an exceptionally fine day. 



To sum up the points considered : 



1. The spectroscope is competent to give trustworthy evidence 

 as to the amount and distribution of aqueous vapor in the 

 atmosphere. 



2. The spectroscope studies vapor height primarily, and 

 humidity only secondarily. 



3. During stormy weather vapor ascends to altitudes greater 

 than is usually supposed. 



4. The great absorption of storm clouds is due to their great 

 thickness, or to extensive strata of damp air associated with 

 them, more than to any peculiar behavior as clouds. 



