Mr. W.S. Jevons on the Cirrous form of Cloud. 23 
- flexures, in a manner altogether forbidding the idea of any re- 
pulsive force between them. Howard also supposes the cirrous 
fibres to act as conductors between masses of dry and moist air 
of opposite electric conditions; but though fibres, once formed, 
might be capable of discharging the electricities with greater 
ease than the surrounding clear air, it does not appear to me 
show the watery particles, which he has moreover to suppose pre- 
cipitated by some other means, come to be gathered up in this 
particular form,—a sort of action with which I know of no 
parallel (a regular polar arrangement being of course a very im- 
probable supposition). 
I have not been able to find that any more satisfactory theory 
has been yet proposed; and, indeed, it would seem that, since 
the establishment of Howard’s arrangement and nomenclature 
of clouds, but little attention has been paid to their study at all, 
particularly as regards the cirrus cloud. 
A simple and natural explanation of these cirrous fibres is, I 
believe, to regard them as minute streamlets of air forcing their 
way through a stratum of air of different temperature and mois- 
ture. We have only to suppose two neighbouring masses of air, 
completely or very nearly saturated with aqueous vapour and of 
different temperatures, to filter into each other in minute stream- 
lets ; and the watery particles which must most certainly be pre- 
cipitated, according to the well-known theory of Dr. Hutton, 
will be arranged so as to present exactly the forms of these 
streamlets, and in fact produce a cirrous cloud. 
The extremely lofty position in the atmosphere at which these 
clouds nearly always occur, renders it impossible for us as yet to 
ascertain even the conditions of the air surrounding them; and 
the untangible nature of a cloud, when reached, would render 
direct experiments upon the mode of its formation entirely out 
of the question. In the case of cirrous clouds especially, we 
can only cbserve with exactness their external character and 
other apparent conditions, and then employ a sort of circumstan- 
tial evidence to demonstrate their nature. 
This I have attempted to do by producing miniature represen- 
tations of clouds, under conditions in which the cause of forma- 
tion could be certainly known. 
The extreme mobility and invisible nature of gases would 
render any experiments upon them, on a small scale, extremely 
difficult and unsatisfactory ; but for our present purposes we 
may, I believe, substitute liquids, for instance water, which 
being so much more sluggish and dense, will be so much the 
slower and more observable in their motions, and will not require 
the same care to prevent accidental disturbances by tempera- 
ture, &e. 
