ANCIENT FLUIDITY OF THE GLOBE. 



363 



fessed, that it is also accompanied with difficulties not easily to be 

 removed. 



If the earth be composed of a solid crust or shell surrounding a 

 fluid mass, this internal fluid would be subjected to the attraction of 

 the sun and moon, or, in other words, would have its regular tides. 

 We are not acquainted with any counteracting influence, to prevent 

 the impulse of these tides upon the solid shell. I am, however, fully 

 persuaded, that the internal parts of the earth do not consist of an as- 

 semblage of chaotic elements, but that they are arranged with as 

 much wisdom as the parts of the external universe, and that the 

 earth itself is the vast laboratory, in which were prepared, according 

 to definite laws, all the mineral substances found on its surface, and 

 in which are now preparing the elements of future changes. There 

 is one difficulty attending the theory of central heat, noticed by Pro- 

 fessor Sedgwick, which it may be proper to state. " If," says he, 

 " during any period the earth has undergone any considerable re- 

 frigeration, it must also have undergone a contraction of dimensions ; 

 and also, as a necessary consequence of a well-known mechanical 

 law, an acceleration round its axis : but direct astronomical observa- 

 tions prove, that there has been no sensible diurnal acceleration dur- 

 ing the last 2000 years ; and, therefore, during that long period, 

 there has been no sensible diminution in the mean temperature of 

 the earth. This difficulty does not, however, entirely upset the pre- 

 vious hypothesis ; it only proves, that the earth had reached an equi- 

 librium of mean temperature, before the commencement of good as- 

 tronomical observations." 



If the terrestrial globe has ever been a fluid ignited mass, it is ob- 

 vious that the atmosphere must have undergone great changes during 

 the progress of refrigeration. In the original ignited state of the 

 earth, all the aqueous particles that form the ocean, and all the more 

 volatile mineral substances, would have existed in the form of va- 

 pour, and have constituted a nebulous medium of vast extent, resem- 

 bling the atmosphere of a comet, or the nebulosity surrounding the 

 newly-discovered planets, Juno, Ceres, and Pallas. By progressive 

 refrigeration, the volatile mineral matter would be concreted, and the 

 aqueous particles precipitated, until the constitution of the atmos- 

 phere became fitted for the support of animal life. It is not improb- 

 able, that the animals of the earliest creation, might have been con- 

 stituted to breathe a denser atmosphere than the present one. Such 

 an atmosphere would, in a considerable degree, equalize the mean 

 temperature of the earth ; and the excess of moisture and of carbo- 

 tiic acid gas, would also be favourable to the rapid development of 

 vegetation. 



In stating these hypotheses, my only object has been to suggest to 

 the reader, the various causes which may have affected the former 

 temperature of the globe, and I shall leave him to determine how far 

 any of them appear to be supported by analogy and probability. 



