^276 Mr. Herapath on the Causes, Laws, and principal [April, 



iie diminished them, thinking there might be a reason for it 

 -which he did not perceive ; but not being thoroughly convinced, 

 he chose rather to omit than describe a calculation, with every 

 part of which he was not perfectly satisfied. This at least 

 appeared to me a very plausible way of accounting for Newton^s 

 silence ; but whether it be a correct one, it is, perhaps, not 

 "worth the trouble of discussing, especially since the complete 

 calculation of this equation, by the celebrated Laplace, in the 

 IW^canique C61este, shows, that whatever may have been New- 

 ton's method, it was, as well as my own, much too loose and 

 inaccurate to be depended on. It is, however, remarkable, that 

 all, or most, of the calculations of the ann. equa. hitherto made 

 from theory, give the quantity of this equation greater than 

 observation ; and this is even the case with Laplace's. 



Having now, as 1 thought, satisfactorily accounted for the 

 JifFerence between Newton's numbers and mine, I became more 

 «trongly persuaded in myself that the tabular magnitude of the 

 nn. equa. was the difference of two equations ; and, therefore^ 

 I frequently tried to unravel the cause and magnitude of the 

 indeterminate one, but without success. At length, about the 

 middle of the following September (1811), my attention being 

 involuntarily turned to a consideration of Newton's opinion 

 respecting the cause of gravitation, I fancied that I saw a true 

 solution of the difficulty in question, as well as a complete deve- 

 lopment of the cause of gravitation. If, argued I with myself, 

 gravitation depends upon the action of an elastic medium, such 

 as Newton supposes, which grows rarer and rarer as you 

 approach the dense bodies of the sun and planets, there ought ta 

 be some reason for this variation of density ; and as Newton ha& 

 not, as far as I could perceive, given any, I began to consider 

 -what it might * be. And after some little thinking, it occurred 

 to me, that if this medium be of the same nature as our atmo- 

 sphere and other gaseous bodies ; that is, if it be capable of 

 heing expanded by heat, and contracted by cold, then, the sun 

 being a very hot body, and the heat being so much the greater 

 the nearer we are to him, the density of the medium oughts 

 therefore, to decrease with a decreasing, and increase with an 

 increasing distance, the same as Newton would have it. And 

 because we find by experience that dense solid bodies receive 

 Tieat more strongly than much rarer ones, particularly^ than 

 gases, the dense bodies of the planets being heated by the solar 

 rays as well as by the medium about them, ought, it appeared to 

 me, to be hotter than this medium, and consequently ought to 

 produce the same effects on the medium as the sun, though not 

 m so great a degree. Therefore if, as Newton imagines, the 



• The only accounts I had seen of Newton*s ideas of this subject were in his Optics, 

 •dtfid at the end of the Principia. I have, however, lately read a letter that he wrote t» 

 Mr. Boyle, printed in Bishop Horsley's edition of his works, wherein he gives his opi- 

 sioD of this tiuid medium a« being of the same nature as our air. 



