630 Mr W. HOPKINS, ON THE EXTERNAL TEMPERATURE OF THE EARTH, 



it, and, under similar circumstances, a portion of the incident heat may pass instantaneously by 

 radiation through an imperfectly diathermanous medium, while the remainder is absorbed by 

 the particles of the medium. 



It would appear improbable that the absorption of light should take place without pro- 

 ducing some physical effect on the absorbing medium, though there are few cases in which we 

 can form any distinct conception of what that effect may be; but we know that light once 

 absorbed is finally extinguished. In like manner heat when absorbed may be destroyed as 

 regards its power of affecting any thermometric instrument, while it produces other effects which 

 the Dynamical Theory of heat has taught us how to appreciate; and, moreover, heat thus 

 absorbed is reproducible as thermometric heat. When heat is first incident on any medium, a 

 portion of it may be employed in producing motion in the particles of the medium or changes 

 in its molecular state ; but when the particles are again reduced to rest, or have assumed a 

 state which, in reference either to their motion or the molecular condition of the medium, may 

 be termed steady, all the heat incident on the medium will be propagated through it as heat, 

 whatever may be the mode by which the transmission may take place. It is this conclusion 

 which assures us that whatever may be the mean rate at which heat is supplied from the 

 Earth's surface to the lower strata of the atmosphere, the same must be the mean rate at 

 which the heat must be transmitted from the upper portions of the atmosphere into surround- 

 ing space ; because the mean state of the general atmosphere, whatever may be the amount of 

 local or periodical changes, must have been permanent for some indefinite period of time, and 

 consequently no heat can ever be permanently absorbed in producing permanent change in the 

 state of the atmosphere. 



The transmission of heat by the last mode above-mentioned, convection, consists in the 

 ascent of the hotter and descent of the colder particles of a medium in which the particles are 

 easily moveable inter se. In the case of the atmosphere, this is one of the dynamical effects 

 of the heat which enters it from the Earth's surface ; but since the mean amount of the vis viva 

 due to this motion, referred to a period of a few years, must be the same for consecutive 

 periods, the remark at the end of the previous paragraph will be applicable to it equally as to 

 other periodical atmospheric changes. 



It must also be recollected that the particular manner in which heat is transmitted does 

 not depend merely on the nature of the transmitting medium, but also on that of the heat 

 transmitted. Glass and atmospheric air, for instance, are diathermanous for the direct solar 

 rays of heat, but uot so for the same heat after it has been absorbed and again emitted by any 

 terrestrial body. It is this property of the atmosphere which renders it so important an agent 

 with reference to the Earth's superficial temperature. 



All the above-mentioned modes of transmitting heat are called into action in the atmo- 

 sphere. It is almost entirely by direct radiation that the solar heat reaches the Earth's 

 surface, when the atmosphere is pure, though a small quantity may even then be absorbed by 

 the atmospheric particles. A considerably greater quantity will, of course, be absorbed when 

 the atmosphere is laden with clouds and vapour. In the transference of the heat back again 

 from the Earth's surface, all the other modes of transmission must be more or less effective, 

 but we are altogether ignorant of their relative degrees of efficiency. For our present purpose, 



