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



stratus, 'Climate of Loudon/ i. p. xlix, says, "The c'lvvi veffetate, 

 as it were, in proportion to the quantity of rain falling." It 

 would be impossible to express more accurately the relation of 

 the subsiding cloud-particles and falling rain to the ascending 

 cirri, than to speak of the latter as " vegetating." 



I leave it for the reader to consider how various winds and 

 many different meteorological conditions will alter the general 

 shape and proportions of the thunder-cloud, and how the cirrose 

 crest will generally be swept away by a lofty cm-rent of air so as 

 to assume the form of a thin sheet or cirrose veil, often of an 

 arched form, and preceding the main body of the storm-cloud. 



Lastly, in proposing, as I think, a fresh and original theory 

 of the thunder-cloud, I am not unaware of its crudeucss. Whether 

 by others or myself, a quantitative estimation must be made 

 of the forces and velocities upon which it so entirely depends ; 

 having suggested that the form of the thunder-cloud is a simple 

 mechanical phjenomenon, it will remain a mere suggestion until 

 it is presented as a rigorous mathematical theorem. The sim- 

 plicity, however, of the principles involved, naturally inspires 

 much confidence that this may be achieved. 



Electrical Theory of the Thunder-cloud. 



30. It may have caused some surprise that I have offered a 

 theoi7 of the thunder-cloud in which the principle, electricity, is 

 not even referred to. 



Thunder and lightning are indeed the most striking phseno- 

 mena which the thunder-cloud presents, and partly on account 

 of this, and partly from a propensity to form electrical theories 

 which seems almost universal, it has always been usual to con- 

 sider that electricity must be, in some way or other, the cause of 

 storms. It is treated, in short, as the fourth element of meteoro- 

 logy, of whichi the first three are air, water, and the agent heat. 



31. If, without assuming that the theories above propounded 

 are exactly true, we only admit that they lie in the right direc- 

 tion, and appear to rest upon some sound basis, we can pretty 

 plainly draw the inference that this supposed electric agency is a 

 mere illusion. All the more distinct forms of clouds, cirrus, 

 stratus, cumulus, cumulostratus, and thunder-cloud, have long 

 been known, or have been shown according to my own theory, to 

 be the simple mechanical effects of differences of specific gravity 

 occasioned in air and water by the action of heat. Electricity 

 is a force of quite a different nature, and incapable of coopera- 

 ting with these in producing the class of mechanical effects in 

 question. To introduce it is gratuitous and unnecessary. 

 Though it is always usual in meteorological works to refer all 

 difficult points concerning clouds to that delusive agent, no one 



