KINETIC THEORIES OF GRAVITATION. 217 



It may be added, that in the fourth book of this labored treatise, the 

 inclination of the ecliptic plane to that of the solar equator and vortex, 

 is supposed to be due to a deviation or drift resulting from the oblong 

 or prolate form of the revolving spheroidal earth."* 



That such an essay should have been crowned by tbe Academy (the 

 prize however being shared between John Bernouilli and his son Daniel 

 as an independent competitor) appears in our day, notwithstanding the 

 mathematical pre-eminence of its author, a somewhat unfavorable illus- 

 tration of the scientific character of the age. Bernouilli has however 

 left us the statement of an elementary truth, which may appropriately 

 close this notice : " In physics we should banish the practice of explain- 

 ing phenomena by chimerical principles more obscure than those pre- 

 sented for investigation."! 



Lesage. 1750. 



Some fifteen years later, another bold scheme of universal impact or 

 pressure, designed to explain and supersede " attraction," was conceived 

 by Georges-Louis Lesage, a French-Swiss physicist and mathematician. 

 By means of an infinite number of " ultramundane corpuscles" of trans- 

 ceudent minuteness and velocity, traversing space in straight lines in 

 all directions, atoms and masses of matter are impelled together differ- 

 entially in the lines of their reciprocal mechanical shadows, or in the 

 direction in which the rectilinear impulses of the " corpuscles" are un- 

 counteracted by opposing ones, from the intervention of other atoms or 

 masses. 



To quote Arago's exposition of the theory, "A single body placed in 

 the midst of such an ocean of moving corpuscles would remain at rest, 

 since it would be equally impelled in every direction. On the other 

 hand, two bodies ought to advance toward each other, since they would 

 form a mutual screen, as their opposed surfaces would no longer be hit 

 in the direction of the line joining them by the ultramundane corpus- 

 cles, and there would then exist currents, the effect of which would no 

 longer be neutralized by opposite currents. Moreover, it will be readily 

 seen that two bodies plunged into such ' gravitation fluid ' would tend 

 to approach each other with a force varying inversely as the square of 

 the distance."! 



Although this scheme presents merely the exchange of one incompre- 

 hensible agent for another, it is perhaps one of the most iugeuious 

 attempts ever made to substitute the conception of primeval motion 

 for that of static tension. 



Lesage was only twenty-three years old, when in 1747 he first devised 



* The measurements of a meridional arc of the earth by James Casini,not long pre- 

 viously, had brought out the curious result that the polar axis of the earth is its long- 

 est diameter. — Traitd de la Grandeur et dela Figure de la Terre. Paris, 1720. 



t Opera : loco ciiat., sec. xxxii, p. 288. 



t Popular Astronomy, book xxiii, chap. 27, vol. ii, p. 4G8. 



