at the Surface of the Earth. W9 



The temperature of the earth will tlier. continue to augment, 

 onlv till the heat which issues from it every moment into the 

 surrounf?ing medium hecome equal to the increase which it re- 

 ceives every moment from the supposed central reservoir. Whcu 

 this happens, the temperature at the superficies can imdergo no 

 further change, and a similar effect must take place with re- 

 spect to every one of the spherical and concentric strata into 

 which we may conceive the solid mass of the globe to be divided. 

 Each of these must in time come to a temperature at which it 

 will give out as much heat to the contiguous stratum on the out- 

 side, as it receives from tl;e contiguous stratum on the inside; and 

 when this happens, its temperature will remain invariable*.'* 



The principle on which this reasoning rests will not be dis- 

 puted. Admitting it therefore, my objects in attempting to nup- 

 port the original argument will be to show, f/st, that such a 

 discharge of heat from the surface of the earth as is here sup- 

 posed docs not take place ; and, secondly, that if it did, this 

 would be as subversive of the system, as if the heat were re- 

 tained. 



There are two modes in which caloric mav he supposed to be 

 conveyed from the surface of the earth; one is by radiation, the 

 other bv slow comamuication by the medium of the clastic fluid 

 which surrounds it. Each of these may be briefly considered. 



The great expansibility and mobility of an elastic fluid, such 

 as the atmosphere, give rise to a peculiarity with regard to the 

 communication of temperature through it. Its proper conduct- 

 ing power appears to be inconsiderable; but whatever it mav be, 

 it is principally by the motion of its parts that heat is difl'used 

 through its mass. When the temperature of a lower portion of 

 such a fluid is raised, it ascends ra])idly; a fresh portion comes 

 in contact with t!ie surface communicating heat; and by this 

 successive application and retrocession of the air, and this move- 

 ment of the heated portion, rather than by the direct commu- 

 riication of caloric from one part to another, the temperature of 

 the whole is raided. It is in this maimer that heat is diffused 

 from the earth into the atmosphere. The air iricumbent on ar.v 

 part of the surface communicating heat, is, by the elevation of 

 its temperature, rarefied, whence an ascending current is formed; 

 and the common opinion with regard to this is, that it carries 

 the excess, of heat to the higher regions of the atmosphere, and 

 may allow it to be diffu-ed into the interplanetary sfsace. It is 

 on the assumption of this that the heat must be suppo-cd, ia 

 Mr. Playfair's argument, to be discharged from the earth, so far 

 at least us it is conveyed from the surface by the surrounding 



* Triinsaclioiis of ilit Sucittv, vol. vi. p. 350. 



clastic 



