THE NAtURAL BISTORY Of GtOUDUV 



Of the Nature of the Cirrus, 



5T 



It xvas necessary to defer the consideration of the nature Nature of th.^ 

 of this cloud, until we had tieveloped, in a considerable de- ^'""^' 

 gree, the principles on which our theory proceeds. The 

 reader will have seen^ that we assume the fact of the slow 

 transnaission of the electric fluid through clouds : which in ' 

 this, as in a former instance, we apply rather analogically 

 than by induction; the modification in question being usu- 

 ally so high in the atmosphere, that the electnc state of the 

 latter, above and below it, cannot easily be found by actual 

 ex|->eriiDei)t.- Proceeding, however, on this assumption, we 

 suppose, that the cirrus resembles in its state a lock of hair, 

 or a feather, insulated and charged; or rather, that its ar- 

 rangements result from the same cause with those of the co- 

 loured powders, which electricians project on a cake of wax, 

 after having touched it with the knob of a chareed phial, 

 and which fall into a variety of configurations on the sur- 

 face. Tluis the cirrus may be formed in the air out of such 

 floatmg particles of water us are present, and may serve the 

 purpose of collecting and trai siiiUting the electric fluid. 

 It is during the prevalence of variable winds, that the cirrus 

 most abounds ; and it is reasonable to conclude, that the 

 portions of air, which at these seasons are transported from 

 place to place, gliding over or intersecting each other, usu- 

 ally differ sufficiently in temperature to occasion a slight de- 

 composition of the vapour of one of the currents, and in 

 their electric charge sufficiently to induce a communication 

 by means of the conducting medium so formed. Again, 

 in the gradual cooling of a perfectly calm plate of air, 

 situate at a great elevation, and consequently free from 

 the occasional causes of disturbance which prevail helow, 

 Tt is not improbable that the separation of the caloric from 

 the vapour, and the collection of the electrised ivater from 

 the air, may go on together, by a process similar to the 

 crystallization of salts, in which much caloric is liberated 

 into the medium. Thin opinion, at least, seems to be ad- 

 vanced by Kirwan, in his *' Essay on the Variations of the 

 Atmosphere," and we may considef the vegetating cirrus as 

 th@ proper example of it. 



Another 



