Mr. W. S. Jevous on the Forms of Clouds. 249 



I believe, that subsidence may go on in the body of a cloud 

 without their ac/grer/ation into vain-drops of sufficient size to de- 

 scend rapidly through the air to the surface of the earth. Thus 

 may be explained, I suspect, the cirrose appearance which 

 Howard {' Climate of London,' vol. i. p. Ixix) detected in the 

 interior of quickly evaporating cumuli. I continually notice, 

 too, that peculiar bag-like projections or " droplets " form at the 

 under surface of a dense sheet of cirrostratus towards the latter 

 part of the day, evidently caused by the weight of subsiding 

 cloud-particles. 



The Nimbus, Rain-cloud, or Thunder-cloud. 



23. The nimbus or cumulo-cirrostratus is au organized com- 

 bination of the simple forms, and is described by Howard as " a 

 horizontal sheet, above which the cirrus spreads, while the 

 cumulus enters it laterally and from beneath." The more 

 distinct forms of thunder-cloud to which I now alone refer, more 

 resemble a cumulostratus with a crest of cirrus spreading from 

 the summit ; and it occurred to me that it was only necessary to 

 combine the conditions of the experiment (see Phil. Mag. for 

 July 1857) by which the miniature cirrus was produced, with 

 the conditions of the miniature cumulostratus, in order to pro- 

 duce an actual representation of the form of a thunder-cloud. 

 The experiment is conducted as follows : — 



2-1. Exp. 3. Pour into the section-glass a stratum of water 

 which contains about 2 parts in 1000 of sugar, with a trace of 

 common salt, and the temperature of which has been raised to 

 about 100° P. Then carefully introduce beneath it a second 

 stratum at the ordinary temperature (about 60°), and containing 

 the same trace of common salt, together with about 2i parts of 

 sugar in 1000. Lastly, project upwards from the lower edge of 

 the apparatus a jet of distilled water at the ordinary temperature, 

 containing merely a trace of nitrate of silver in solution. 



25. The jet of the last-named liquid forces its way rapidly up 

 through the lowest cold and dense stratum, by virtue both of its 

 momentum and its considerable buoyancy. The heated upper 

 stratum, however, although containing sugar, is of less specific 

 gravity than pure water at the temperature of 60°, and therefore, 

 on entering this stratum, the upward progress of the jet is 

 checked, and a cumulostratose form, similar to that of the last 

 experiment, is the first result. But the heat of the uppermost 

 stratum is rapidly communicated to the liquid with which it is 

 in contact ; and wherever there are two portions of liquid of the 

 same temperature, and of which the up])er contains sugar, while 

 the lower is free from it, it is evident that these must change 

 place from the mere effect of gravity. 



