6 



Mr. G. J. Stoney on the Physical 



[Recess, 



terval, we may suppose that the part of the atmosphere which occupies it 

 is either nearly saturated with the vapour from which clouds are formed, 

 or but sparingly supplied with it. If it be very moist, the clouds as they 

 descend, either through convection or by subsidence, will pass into the 

 form of mist, which will collect into a rain that will fall towards the ocean 

 beneath. If, on the other hand, the interval between the clouds and ocean 

 be far from being charged with vapour, the cloud as it descends will dis- 

 solve away among the hot and dry gases below, while the ascending cur- 

 rents, as they rise into the situation from which they can freely radiate, 

 restore the same quantity of a thin gauze-like cloud. The possible alter- 

 natives, then, are, 1st, that the interval between the clouds and ocean is 

 transparent, or, 2nd, that it is rendered in a considerable degree opake by 

 mist and rain, or, 3rd, that the clouds reach to the ocean. 



13. Let us examine these hypotheses, beginning with that of a clear 

 atmosphere under the clouds. If there be such a transparent space, 

 it is easy to see that the intensity of the rays which strike the under 

 surface of the clouds is greater than that part of the solar radiation 

 which emanates directly from the clouds ; for a portion of the rays which 

 flow downwards directly from the clouds will be reflected by the body of 

 the sun beneath ; another portion will be scattered at the same surface. 

 These two portions will fall short of the entire quantity of rays in the first 

 instance radiated downwards from the clouds. But as the body of the 

 sun is hotter than the clouds, what is here wanting will, according to a 

 known law regarding the exchange of heat, be more than made up by what 

 the body of the sun will itself emit*. Thus we have already a quan- 

 tity of heat radiated upwards against the clouds greater than that emitted 

 by them downwards. But further, the clouds must scatter a part of the 

 rays that strike their under surface. Some of the rays so scattered will 

 be afterwards reflected or scattered by the body of the sun, and will 

 augment still further the heat striking the under surface of the clouds 

 over that which they radiated downwards. This excess will be great if the 

 clouds be of a material that scatters light copiously ; for in this case we 

 shall not only have a large supply of rays that had been so scattered 

 added to the stock, but also, if the clouds scatter incident rays abundantly, 

 they will, in obedience to the laws of the exchange of heat, be at the same 

 time such as will emit more feebly ; and accordingly the brightness which 

 shines upon them from the background will be relatively more conspi- 

 cuous. But, however this may be, whether the excess be more or less, it 



* In fact, if a be the proportion of incident rays reflected by the molten ocean, and 

 /3 be the proportion scattered by it, and if A be the quantity of heat which would be 

 emitted per square metre by the surface of a perfect radiator as hot as the molten 

 ocean, then will the quantity emitted by the molten ocean be (1 — a— /3) A. 



But B, the quantity sent down by the clouds which is incident on a square metre of 

 the ocean, is less than A, since the clouds are at a lower temperature ; and of this, 

 (a+/3) B is returned upwards. Adding, we find the whole quantity sent upwards to be 

 — a — j3) (A— B), which is greater than B. 



