368 Mr. S. T. Preston on some Di/namical Conditions 



out the greatest range in wliich we have observed them, which 

 is but an infinitesimal fraction of the distance of the stars *. 



9. It may perhaps be well just to sketch here the mode of 

 action of the medium in producing gravity, the manner in which 

 the intensity is made to vary as the square of the distance, &c. 

 Let A (in the annexed diagram) represent a molecule or mass; 

 let C represent the bounding sur- 

 face of an imaginary hollow sphere 

 described about A. Then, since the 

 particles of the medium are moving 

 uniformly in all directions, a number 

 of them will be passing in all direc- 

 tions through the imaginary sphe- 

 rical surface C. Only those par- 

 ticles which are passing (sensibly) 



along the radii of the spherical surface will strike A ; and 

 therefore we need only regard those special particles which 

 radiate towards A. The molecule A being therefore struck 

 equally on all sides, will accordingly remain at rest. But if now 

 we suppose a second molecule to be placed at B, then out of 

 the whole number of particles which are directed towards A, 

 the molecule B will intercept a number which is proportional 

 to the area which B cuts off from the whole spherical area C. 

 The molecule A wdll therefore now, owing to the sheltering 

 power of B, be struck wdth a fewer number of particles in the 

 direction B A. The balance of the pressure being thus upset, 

 A will be propelled tow^ards B. The same holds true of B 

 relatively to A (on drawing the imaginary spherical sur- 

 face C^). The two molecules A and B are therefore pro- 

 pelled towards each other mutually. It now remains to illus- 

 trate how the impulsive action varies as the square of the dis- 

 tance. It will be at once evident that since the area of a 

 spherical surface is as the square of the radius, therefore, if B 



* It may just he noted in connexion with this, that if tlie above deduc- 

 tion as to the stars not grayitating towards each other he true, those stars 

 which have a proper motion must be moving in straigld lines. 



It is of course evident that all the relations (admirable in their simpli- 

 city) existing between the velocity of the particles, the pressure, and 

 density of the medium (or quantity of matter in unit volume), the dimen- 

 sions, distance, and mean path of the particles, which apply to the kinetic 

 theory of gases, apply equally to the gravific medium, — as also the physical 

 relation existing between the velocity of the particles and the Telocity of 

 a wave in a gaseous body, pointed out by me (Phil. Mag. June 1877), the 

 numerical value of which (as determined by Professor Maxwell) was given 

 in the above paper. The assumption is perhaps not unwarranted that it 

 may eventually be possible, by determining the absolute value of some of 

 the above relations, to calculate the mean path, and therefore the range of 

 gravity. 



