244 Mr. W. S. Jevons 07i the Forms of Clouds. 



nomena which may be produced by the experimental arrange- 

 ments described above, result from only two essential variable 

 conditions. 



1st. Original impressed momentum. 



2nd. Gravity, which is called into play whenever we use two 

 liquids differing, however slightly, in specific gravity, but which 

 does not in the least affect the internal motions of a peifectly 

 homogeneous fluid. 



Though we employ certain chemical salts and also liquids dif- 

 fering in temperature, it will easily be seen, and must be borne 

 in mind, that these are merely devices for rendering the motions 

 of the liquids visible, or for altering their specific gravities. 



8. In the use of the section-glass, many curious facts have 

 been observed concerning the movements of jets and streams in 

 the interior of a body of liquid, which this is not the place to 

 describe, although the great variety of gracefully curved lines, 

 and of curious and complicated forms which are presented, give 

 a lively interest to the experiments. In a few of the more di- 

 stinct cases, I hope to show that they lead us to the understand- 

 ing of analogous atmospheric movements on a much grander 

 and more important scale. 



Cumulus. 



9. The general nature and principles of the cumulus have 

 been understood for some time past ; but as the explanations 

 which are given of it in meteorological works are, I believe, more 

 or less erroneous, and always incomplete, I must here state what 

 occurs to me concerning it, before proceeding to the more com- 

 plex cases of the cumulostratus and nimbus. 



10. The pure cumulus is a simple, roundish, or hemispherical 

 heap of cloud matter, extending itself upwards from a horizontal 

 base, and appearing mysteriously suspended quite apart from all 

 other clouds at a moderate distance from the surface of the earth. 

 In this pure form it occurs only during the day ; and this fact 

 leads to the supposition, well supported by the observation of its 

 general form, and the curious internal motions which it often 

 exhibits, that it is occasioned by upward currents of warm and 

 moist air originating at the surface of the earth. 



11. ^Vlien a body of air rises in the atmosphere, the continual 

 diminution of the superincumbent pressure allows it to expand, 

 and to decrease in density at about the same rate as obtains in 

 every other part of the atmosphere. Expansion occasions the 

 increase of the capacity for heat, and there is consequently an 

 absorption of sensible heat. The rising air must therefore sink 

 in temperature as it swells in volume ; but because a correspond- 

 ing expansion of volume and diminution of density takes place 



