Mr. W. S. Jevons on the Forms of Clouds. 245 



in every other part of the atmosphere, a uniform corresponding 

 fall of temperatm'e must also be everywhere found, supposing no 

 extraneous disturbing influences to exist. Thus the walls of air 

 which sm-round the ascending column, and were cold with regard 

 to it at the surface, must suffer in every part of its course an 

 equal diminution of temperature proportional to the height at 

 which we test it. In fact, however high the column of air ascend, 

 it will fall continuously in temperature, but still remain warm 

 with respect to the surrounding air on the same level. Its 

 motion, supposing it not to become broken up and dispersed, 

 might thus be perpetuated up to the limiting surface of the 

 atmosphere but for unforeseen causes, or perhaps for the inter- 

 ference of the contained aqueous vapour. 



12. It is evident that the aqueous vapour, which is always 

 entangled in greater or less quantity with the ascending air, must 

 experience exactly corresponding changes of volume and density. 

 In proportion as the air expands, therefore must the aqueous 

 vapour diminish in actual elastic force. \^\iq actual elastic force 

 is a term used in distinction from the possible elastic force, by 

 which is meant the greatest tension, according to the usual 

 tables, that can be supported at any given temperature.] But 

 since the temperature of a rising body of air falls at some uni- 

 form rate, or at all events proportionally to the expansion, while 

 the possible elastic force of vapour decreases in some ratio much 

 more rapid than the temperature, it follows that the possible will 

 always, sooner or later, overtake the actual elastic force of the 

 contained aqueous vapour. If the upward motion of the air be 

 continued ever so little above the point at which this takes place, 

 the actual elastic force can be no longer maintained, but con- 

 densation must ensue and a cloud be produced. 



13. It now becomes interesting to inquire whether or not the 

 ascending current will be continued to any considerable height 

 above that " j)lane of precipitation" at which, as just explained, 

 condensation of aqueous vapour commences. As yet we have 

 found no reason (11) why the motion should not be continued 

 up to the surface of the atmosphere, while it is commonly stated 

 that the large amount of latent heat given out in condensation 

 tends even to increase the buoyancy and " perpetuate " the up- 

 ward movement (Graham^s 'Elements,' p. 328). 



Yet it is evident that the uprising column of air is actually 

 terminated, for the reason that the cumulus, as continually occur- 

 ring in the atmosphere, is always of a moderate elevation, and 

 terminates in a sharply defined spherical head. To judge solely 

 from the ap])earance of the cloud itself, we should at once say that 

 just above that level where condensation of vapour commences, the 

 rising column is suddenly checked, flows over in a fountain-like 



