322 Oh the Mean Density of the Earth. 



Although it appears by the foregoing letter from the Marquis 

 De Laplace, that his ^iemoir on the earth's density will shortly 

 appear in the Con/ioissance des Te7m, yet tiie learned world 

 must be gratified with an earlier publication of it, especially in 

 your valuable Magazine, which has so extensive a circulation ; 

 I therefore send you the following translation. 



Your readers will observe, in the Memoir, that elegant sim- 

 plicity and perspicuity for which all the other works of the learned 

 author are distinguished. The tribute which he pays to our 

 immortal countryman is as eloouent as it is true, and must be 

 read with the deepest interest. No man more highly appreciates 

 the discoveries of Sir Isaac Newton than the Marquis De La- 

 place, and no philosopher has better illustrated them. Thus 

 the fame of Newton increases with the progress of science and 

 of time, verifying the apposite cjuotation from Cicero, at the con- 

 clusion of the Memoir, which may be thus freely translated, 

 " Time, which obliterates fanciful theories, confirms decisions 

 founded on nature." 



On the Mean Density of the Earth. By M. De Laplace. 



One of the most curious questions in geology, is the ratio of 

 the mean density of the terrestrial spheroid to that of a known 

 substance. Newton, in his Mathematical Principles of Natural 

 Philosophy, gave the first idea ever published on this subject. 

 This admirable work contains the elements of all the great dis- 

 coveries that have been since made in the system of the universe. 

 The history of their development by the successors of this great 

 geometrician, would be at once the most useful commentary on 

 his works, and the best guide to new discoveries. The following 

 passage of his work, which relates to the subject in question, 

 will be found in the different editions of that work : 



" I thus assume; that the terrestrial globe is more dense than 

 water; if it were entirely formed of it, all rarer bodies would 

 rise and float on the surface,— their specific gravity being less. 

 Thus, supposing the globe of the earth to be entirely covered by 

 water ; if it were more rare than the water, some part would 

 discover itself, and the waters of the parts uncovered would col- 

 lect in the opposite region. The same thing must take place as 

 to our earth, which is in a great part covered by the ocean. If 

 it were less dense than the water, it would rise on account of its 

 lightness ;— the waters flowing towards the opposite regions. 

 For the same reason, the spots on the sun are lighter than the 

 luminous matter in which they float; and in the formation of 

 the planets, whatever it may have been, the most dense matter 

 was carried towards the centre, when the whole mass was fluid. 



Thus, 



