mth the Condensation of Vapour. 1 69 



Mr. Howard, in his remarks on the formation of nimbus 

 (Introduction, page Ixxi), a cloud very closely allied to the 

 cumulostratus, expresses himself in almost the precise words 

 of the Committee of the Royal Society*. "The cirri, also, 

 which so frequently stretch from the superior sheet upwards 

 and resemble erected hairs, carry so much the appearance of 

 temporary conductors o^ the electricity extricated by the sudden 

 union of its minute drops into the vastly larger ones that form 

 the rain, that one is in a manner compelled, when viewing the 

 phaenomenon, to indulge a little in electrical speculation." The 

 sudden agglomeration alluded to by the Committee is here 

 unmistakeable; and it would appear from the course of rea- 

 soning developed in the preceding extracts, that the greater 

 manifestation of the electrical attraction — noticed under the 

 head of "formation of cumulus" — consequent upon the coa- 

 lescence or union of the cumulus with the cirrostratus, which 

 is generally crowned with cirrus, is most probably — especially 

 when combined with the pressure, in the lower part of the 

 cloud, of the rising ; and the deposition on its upper surface 

 of the descending vapour — the occasion of the disturbance of 

 the electrical state of the cumulus by which the rain is pro- 

 duced. We have, in fact, the entire process beautifully pre- 

 sented to our notice, commencing with the incipient separa- 

 tion of the earliest drops of water by the agency of a dimi- 

 nished temperature from the vapour rising above the vapour 

 plane, which drops are feebly electrified and more or less per- 

 fectly insulated ; the process terminating with the sudden ag- 

 glomeration of these minute and feebly electrified drops into 

 the vastly larger ones that form rain, by which the electric 

 tension may be, and doubtless is, so enormously increased, 

 that the electric state of the cloud and of bodies in its imme- 

 diate neighbourhood are extensively disturbed ; so that if the 

 superabundant electricity should not be carried off by the 

 conducting or rather transmitting crown of cirri, as suggested 

 by Howard, the exhibition of electrical phaenomena, as thunder 

 and lightning, the stroke being given either by the cloud or 

 the rain, generally follows. The intermediate steps appear to 

 be governed by the "two grand predisposing causes — a falling 

 temperature and the influx of vapour." Should these not be 

 sufficient in their extent, either above or below, to produce a 

 formation of cirrus or an increase of cumulus, the individuals 

 of the latter modification gradually evaporate as evening ap- 

 proaches, and generally give place to a serene and tranquil 

 night ; but if both these classes of cloud form rapidly, cirro- 

 stratus generally shows itself, electrical action is called more 

 * Philosophical Magazine, vol. xxxv. p. 161, foot note. 



