40 



Mr. G. J. Stoney on the Physical 



[Recess, 



sides of the two arcs shown in the photographs exhibit such a ruggedness 

 that, as Dr. De La Rue has pointed out, it cannot be accounted for by the 

 mountainous edge of the moon. In fact the film of cloud seems to be so 

 excessively thin that even during an eclipse it can only be seen where it is 

 presented very nearly edgewise at the extreme margin of its disk, or for a 

 short distance inside it, a distance which varies with the local density of 

 the film, and so gives rise to the appearance in question. 



67. This second shell of clouds, as they consist of the same materials as 

 the clouds of the photosphere, and are higher in the atmosphere, and 

 therefore subjected to less pressure, will evidently not form until they can 

 do so at a somewhat lower temperature. But the difference may be so 

 slight that in their normal position these clouds lose more heat by radia- 

 tion towards the sky than they receive by absorption from the photo- 

 sphere, which would cause them to imitate, but with a languor propor- 

 tional to their flimsiness, all the phenomena of convection, &c. which we 

 have traced in the principal layer of clouds. 



68. But this behaviour would be altogether changed if by any cause a 

 part of the film were borne upwards into the cool regions above. At what- 

 ever part of the atmosphere a cloud may find itself, it will be exposed to 

 the unmitigated glare of the photosphere, and will be raised by it to a 

 temperature bordering upon that of the photosphere itself*. A cloud in 

 this situation will therefore warm, instead of cooling, the air in which it is 

 dispersed, and will tend to float violently upwards until it gets to a part of 

 the atmosphere so rare, that the particles of condensed vapour tend to sink 

 in it from their specific gravity as fast as they are carried upwards by the 

 body of heated air entangled with them. This may be the cause of the 

 columnar clouds with overhanging tops which have heen observed during 

 eclipses. As they spread out at the top and become diffused, they will 

 not as effectually heat the intermingled air, and will therefore begin to sub- 

 side. Between clouds that are carried so violently upwards and those that 

 repose in the luminous shells, any intermediate descriptions may exist, 

 and were perhaps the cause of the mountainous projections from the upper 

 shell that have been seen, and of several of the detached clouds. 



69. But besides the materials that enter into the composition of the 



* If we could trust at high temperatures, which of course we cannot, Dulong and 

 Petit's law for the velocity of cooling, viz. : — 



v=k(at— at'), 



where v is the fall of temperature per unit of time ; 

 tf, temperature of the particle in Centigrade degrees ; 



t', the temperature of the radiations to which it is supposed to be subjected on all sides ; 



a constant, depending on the nature of the particle and on the position we assume 



as the zero of our thermometric scale ; and 

 a = 1-0077; 



we should find that the temperature of a cloud exposed, on one side to the photosphere, 

 and on the other to the sky, falls short of the temperature of the photosphere by little 

 more than 90° Centigrade. 



