ATTRACTION. 489 



and the other negative, but are in respect of gravitation all of the same kind, 

 and that the force between them is in every case attractive. To account for 

 such a force by means of stress in an intervening medium, on the plan adopted 

 for electric and magnetic forces, we must assume a stress of an opposite kind 

 from that already mentioned. We must suppose that there is a pressure in 

 the direction of the lines of force, combined with a tension in all directions 

 at right angles to the lines of force. Such a state of stress would, no doubt, 

 account for the observed effects of gravitation. We have not, however, been 

 able hitherto to imagine any physical cause for such a state of stress. It is 

 easy to calculate the amount of this stress which would be required to account 

 for the actual effects of gravity at the surface of the earth. It would require 

 a pressure of 37,000 tons weight on the square inch in a vertical direction, 

 combined with a tension of the same numerical value in all horizontal directions. 

 The state of stress, therefore, which we must suppose to exist in the invisible 

 medium, is 3000 times greater than that which the strongest steel could sup- 

 port. 



Another theory of the mechanism of gravitation, that of Le Sage, who 

 attributes it to the impact of "ultramundane corpuscles," has been already 

 discussed in the article Atom, supra, p. 473. 



Sir William Thomson* has shewn that if we suppose all space filled with 

 a uniform incompressible fluid, and if we further suppose either that material 

 bodies are always generating and emitting this fluid at a constant rate, the 

 fluid flowing off to infinity, or that material bodies are always absorbing and 

 annihilating the fluid, the deficiency flowing in from infinite space, then, in 

 either of these cases, there would be an attraction between any two bodies 

 inversely as the square of the distance. If, however, one of the bodies were 

 a generator of the fluid and the other an absorber of it, the bodies would repel 

 each other. 



Here, then, we have a hydrodynamical illustration of action at a distance, 

 which is so far promising that it shews how bodies of the same kind may 

 attract each other. But the conception of a fluid constantly flowing out of a 

 body without any supply from without, or flowing into it without any way 

 of escape, is so contradictory to all our experience, that an hypothesis, of 

 which it is an essential part, cannot be called an explanation of the phenomenon 

 of gravitation. 



* Proceedings of the Royal Svciety of Edinburgh, 7th Feb. 1870. 



VOL. II, 62 



