1-lrt HISTORY OF THERMOTICS. 



cients are to be determined for each case by appropriate experiments : 

 when the experimenters had obtained these data, as well as the ma- 

 thematical solutions of the problems, they could test the truth of their 

 fundamental principles by a comparison of the theoretical and actual 

 results in properly-selected cases. This was done for the law of con- 

 duction in the simple cases of metallic bars heated at one end, by M. 

 Biot, 7 and the accordance with experiment was sufficiently close. In 

 the more complex cases of conduction which Fourier considered, it was 

 less easy to devise a satisfactory mode of comparison. But some 

 rather curious relations which he demonstrated to exist amono- the 



O 



temperatures at different points of an armille, or ring, afforded a good 

 criterion of the value of the calculations, and confirmed their correct- 



ness. 8 



We may therefore presume these doctrines of radiation and con- 

 duction to be sufficiently established ; and -we may consider their 

 application to any remarkable case to be a portion of the history of 

 science. We proceed to some such applications. 



Sect. 4. The Geological and Cosmoloyical Application of Thcrmotics. 



Bv far the most important case to which conclusions from these doc- 

 trines have been applied, is that of the globe of the earth, and of 

 those laws of climate to which the modifications of temperature give 

 rise ; and in this way we are led to inferences concerning other parts 

 of the universe. If we had any means of observing these terrestrial 

 and cosmical phenomena to a sufficient extent, they would be valuable 

 facts on which we might erect our theories ; and they would thus form 

 part, not of the corollaries, but of the foundations of our doctrine of 

 heat. In such a case, the laws of the propagation of heat, as discovered 

 from experiments on smaller bodies, would serve to explain these phe- 

 nomena of the universe, just as the laws of motion explain the celes- 

 tial movements. But since we are almost entirely without any definite 

 indications of the condition of the other bodies in the solar system as 

 to heat; and since, even with regard to the earth, we know only the 

 temperature of the parts at or very near the surface, our knowledge of 

 the part which heat plays in the earth and the heavens must be in a 

 great measure, not a generalization of observed facts, but a deduction 

 from theoretical principles. Still, such knowledge, whether obtained 



T Tr. dsPhys. iv. 671. " Mem. lust. 1819, p. 192, published 1824. 



