1821.] Phcmomena of Heaty Gases, Gravitation, S^c. 277 



particles of the planets be impelled towards the sun by the ine- 

 quality of pressure on their further and nearer sides, the denser 

 parts of the medium pressing more forcibly than the rarer, the 

 same reason will likewise hold good why bodies should be 

 impelled towards the planets and other material parts of the 

 system. 



And by considering these things further, it seemed to me tliat 

 if such be the cause of gravitation, the intensity of the impelhn^ 

 force should be subject to the influence of two circumstances ; 

 namely, the number of particles in the central body, and their 

 . temperature ; so that it becomes greater when either of these 

 becomes greater, and less when it becomes less. But since the 

 earth in its passage round the sun is sometimes at a greater and 

 sometimes at a less distance from it ; and since the regions- 

 which are nearer the sun are hotter than those which are more 

 remote, the temperature, and consequently the attraction of the 

 earth, should increase as the earth approached the sun, and. 

 diminish as it receded from it, so as to be greatest about the 

 perihelion, and least about the aphelion. And this being the 

 case, the moon must move in a contracted orbit, and swifter 

 round the perihelion earth, and in a dilated orbit, and slower 

 round the aphelion earth ; by which means an equation to the 

 moon's mean motion must be generated contrary to the ann. equa. 

 and diminish it, the same as I had supposed some unknowa 

 equation ought to do with the theoretical annual equation to 

 reduce it to the tabular. , 



Thus it happened that the inadequate method of computatioa 

 I had adopted, brought out a quantity which so well accorded 

 with my first theoretical views of the cause of gravitation, that I 

 could not help placing great confidence in the theory I had 

 embraced. I, therefore, carried on my speculations with that 

 ardour which a strong prejudice in favour of the truth of my 

 principles, and the sanguine hopes of succeeding in so great a 

 problem as that of developing the cause of gravitation, might- 

 naturally be supposed to inspire ; but 1 soon found that before 

 I could proceed any further, I must estabhsh the cause of heat^ 

 and reduce its phainomena to mathematical laws. This I at first 

 attempted to do by endeavouring to find out the relation which 

 should exist between the masses of the particles of the ethereal 

 medium and their repulsive force, in an equation connected with, 

 their distances from the sun. But being disappointed in this, 

 and a great number of other attempts that 1 made, I became 

 much dispirited, and was often on the point of forming a reso~ 

 lution never to consider the subject again. Indeed I frequently 

 wished to persuade myself that the discovery was altogether 

 beyond the reach of human ability ; and with this view tried to 

 thrust it entirely from my mind. Yet sometimes, when my 

 thoughts were involuntarily turned this way, the idea that two 



