DELATION OF VAPOR AJS"D AIR. 179 



Parhelia, Corona?, Glories, and the like ; Electricity, in the explana- 

 tion of Thunder and Lightning, Hail, Aurora Borealis ; to which others 

 might be added.] 



Clouds. When vapor becomes visible by being cooled below it.s 

 constituent temperature, it forms itself into a very fine watery powder, 

 the diameter of the particles of which this powder consists being very 

 small : they are estimated by various writers, from l-100,000th to 

 l-20,000th of an inch. 2 * Such particles, even if solid, would descend 

 very slowly ; and very slight causes would suffice for their suspension, 

 without recurring to the hypothesis of vesicles, of which we have 

 already spoken. Indeed that hypothesis will not explain the fact, 

 except we suppose these vesicles filled with a rarer air than that of 

 the atmosphere ; and, accordingly, though this hypothesis is still 

 maintained by some, 26 it is asserted as a fact of observation, proved by 

 optical or other phenomena, and not deduced from the suspension of 

 clouds. Yet the latter result is still variously explained by different 

 philosophers : thus, M. Gay-Lussac 2t accounts for it by upward cur- 

 rents of air, and Fresnel explains it by the heat and rarefaction of air 

 in the interior of the cloud. 



Classification of Clouds. A classification of clouds can then only 

 be consistent and intelligible when it rests upon their atrnological con- 

 ditions. Such a system was proposed by Mr. Luke Howard, in 1S02-3, 

 His primary modifications are, Cirrus, Cumulus, and Stratus, which 

 the Germans have translated by terms equivalent in English io feather- 

 cloud, heap-cloud, and layer-cloud. The cumulus increases by accumu- 

 lations on its top, and floats in the air with a horizontal base ; the stra- 

 tus grows from below, and spreads along the earth ; the cirrus consists 

 of fibres in the higher regions of the atmosphere, which grow every 

 way. Between the simple modifications are intermediate ones, cirro- 

 cumulus and cirro-stratus; and, again, compound ones, the can' 

 stratus and the nimbus, or rain-cloud. These distinctions have been 

 generally accepted all over Europe : and have rendered a description 

 of all the processes which go on in the atmosphere far more definite 

 and clear than it could be made before their use. 



I omit a mass of facts and opinions, supposed laws of phenomena 

 and assigned causes, which abound in meteorology more than in any 

 other science. The slightest consideration will show us what a great 



6 Kfemtz, Jfet. i. 393 - 5 Ib. i. 393. Kobison, ii. 13 



T J/IH. Cftim. xxv. 1822. 



