8 On the Modifications of Clouds, arid 



librium among the particles may be restored at one exfi'ie- 

 mity some tmie before the other has ceased to transmit j 

 vvhence a visible progress of the change which may be traced 

 in a cirrus of sufficient length^^ 



That an extensive horizontal cirnt* should become divi- 

 ded across its ramifications, and that these divided parts 

 should assvime more or less of a round form, is also con- 

 sistent with the idea of a change of this sort*. It is not 

 so easy to give a reason why these small orbicular masses 

 slvoukl remain in close arrangement, or even m contact, for 

 several hours, forming a svstem of small clouds which yet 

 do not interfere with each other or run together into one, 

 but remain as it were in readiness to reform the cirrus-, 

 which sometimes happens very suddenly, though they more 

 frequently evaporate by degrees. 



Ilie same remark must be applied to the curious and 

 as it were capricious divisions and subdivisions, both lon- 

 gitudinal and transverse, which happen in the cirro-stratus 

 when this cloud is verging towards the cirro-cumulus. In 

 general, nevertheless, its appearance is s-ufficiently distmct 

 from that of the cirrus and cirro-cumulus. The cirrus by 

 its great extent in proportion to its mass, its distinct lines 

 and angular flexures in all directions, and the cirro-cumulus 

 bvthe r(;undnes3 and softness of its forms, indicates an es- 

 sential difference in the state of the containing atmosphere. 

 The cirro-stratus appears to be alwavs in a subsiding state, 

 slowlv diminishing by evaporation or dispersion, and at the 

 same time more feebly acted upon by electricity than the 

 preceding modifications. Indeed, the lower atmosphere is 

 usually pretty much charged with dew or haze at the time 

 of its appearance, and therefore in a state to conduct it to 

 the earth. 



Of the Nature of Ike Compound Modifications, and of the 

 Resolution of Clouds into Rain, &'c. 



From the theory of evaporation it appears that no per- 

 manent cloud can bif* formed' in the atmosphere, however 

 low the temperature, without -a sufficient pressure from va- 

 pour prenously diffused. Hence, although in cold weather 

 the breath and perspiration of animals, as also water at a 

 certain excess of teinperature, occasion a visible cloud, and, 



• A quickly evaporating cumi'lus sometimes leaves a regular cirnis 

 beh'nd, formed out of tlie remnant of the cloud whicli in tl:c iiircrmt- 

 diiitt state, and just when it bezjins to show the sky through it, ex- 

 actly represents the poas and fibre of sponge. 'I his may be attributed 

 ta the quantity of eltctrioity passing into or frotii tht cioud at that time 



in 



