CHAPTER OF VARIETIES. 123 



canic action appears to be related (and this is a subject which has 

 long occupied M. Cordier, though he has considered it under ano- 

 ther point of view) to the high temperature now existing in the 

 interior of the globe. 



Now the secular refrigeration, that is to say, the slow diffusion of 

 the primitive heat to which the planets owe their spheroidal forms, 

 and the generally regular disposition of these beds from the centre 

 to the circumference, in the order of specific gravity, the secular 

 refrigeration, on the march of which M. Fourier has thrown so much 

 light, does offer an element to which these extraordinary effects may 

 be referred. This element is the relation which a refrigeration so 

 advanced as that of the planetary bodies establishes between the 

 capacity of their solid crusts and the volume of their internal 

 masses. In a given time, the temperature of the interior of the 

 planets is lowered by a much greater quantity than that on their 

 surfaces, of which the refrigeration is now nearly insensible. We 

 are, undoubtedly, ignorant of the physical properties of the matter 

 composing the interior of these bodies, but analogy leads us to con- 

 sider, that the inequality of cooling above noticed, would place their 

 crusts under the necessity of continually diminishing their capaci- 

 ties, notwithstanding they should not cease to embrace their inter- 

 nal masses exactly, the temperature of which diminishes sensibly. 

 They must, therefore, depart in a slight and progressive manner, 

 from the spheroidal figure proper to them, and corresponding to a 

 maximum of capacity ; and the gradually increased tendency to re- 

 vert to that figure, whether it acts alone, or whether it combines 

 with other internal causes of change which the planets may contain, 

 may, with great probability, completely account for the ridges and 

 protuberances which have been formed at intervals on the internal 

 crust of the earth, and probably also of all the other planets, 



CHAPTER OF VARIETIES. 



CLOUD ECHOES AND THE ROLLING OF THUNDER. The rolling of 

 thunder has been attributed to the echoes among the clouds j and if 

 it is considered that a cloud is a collection of particles of water, how- 

 ever minute, yet in a liquid state, and therefore each individually 

 capable of reflecting sound, there is no reason why very loud sounds 

 should not be reverberated confusedly (like bright lights) from a 



