248 Mr. W. S. Jevous un the Furms of Clouds. 



redescend, as in the cumulus, into the lower stratum. But here 

 its downward motion will be again checked, so that in fact it 

 cannot attain equilibrium except by spreading out in the form 

 of a distinct layer or stratose extension intermediate to the other 

 two strata. A cloud of chloride of silver is thus produced of 

 which two of the varieties of form are shown in figs. 3 and 4, 

 being drawn from photographs of the actual experiments. 



20. Assuming that the resemblance of the miniature and real 

 atmospheric cloud is sufficient ground, I can at once announce, 

 from the conditions of the experiment, that the cumulostratus is 

 a cumnlus in which the tendency to redescend beneath the plane 

 of precipitation does not exist. On the contrary, the aii", after 

 ascending and becoming cloudy, finds itself in equilibrium with 

 the surrounding air, and spreads out in more or less elevated and 

 horizontal projections. But I must remark, that the miniature 

 cloud having once suggested the conditions of the real cloud, 

 ceases to be the sole ground on which we accept these conditions 

 as true. The mind having once received a clear and salient idea 

 as to a probable cause of the cumulostratus, will apply it to every 

 instance of that cloud which meets the eye in nature. This com- 

 parison is the test of the theory, and must be the main gi'ound 

 on which my readers may judge of the truth of my conclusions. 



21. Having established the simple dynamical conditions of 

 the cumulostratus, it yet remains, as before explained, to trans- 

 late these conditions, and to show the ultimate physical causes 

 of the cloud. One simple and apparently reasonable supposition 

 is sufficient, viz. that a partial subsidence or separation of the 

 watery cloud-particles takes place. The evident effect of this is 

 to diminish the specific gravity of the remaining cloudy air. 

 The latter was, in the first place, warmer than the surrounding 

 air when it ascended, and became furtlier warmed by the heat 

 evolved in condensation. Its buoyancy, however, was destroyed, 

 and its upward motion arrested above the plane of precipitation, 

 by some causes to which the weight of the watery particles un- 

 doubtedly contributed in a greater or less degree. But when 

 the watery particles fall away from the air, a part at least of this 

 counteracting tendency is removed; the VLwhtcovae.^ permanently 

 warmer and permanently lighter than bcibre, and will not have 

 the same tendency to redescend beneath the plane of precipita- 

 tion, as in the cumulus. Still retaining a part of the cloudy 

 particles in suspension, it will seek a certain elevation at which 

 it is in equilibrium with the surrounding air, and will thei'e 

 spread out in those stratiform projections which characterize the 

 cumulostratus. 



22. The separated watery particles may fall as rain ; but this 

 is not a necessary supposition, since many appearances indicate. 



