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Leonard Euler, the eminent Swiss mathematician and philosopher, (a pupil of Beruouilli previously referred to,) entertained an indefinite impression that the setherial medium is in some way a connecting link between the celestial bodies, inducing that mutual tendency to approach commonly called "attraction." Only some dozen years later than the date of Lesage's first conception, he briefly discussed the subject in his celebrated "Letters" commencing in 1760.

He thus comments on the action of gravity : " Supposing a hole made in the earth through its center; it is clear that a body at the very center must entirely lose its gravity, as it could no longer move in any direction whatever, all those of gravity tending continually toward the center of the earth. Since then a body has no longer gravity at the center of the earth, it will follow that in descending to this center, its gravity will be gradually diminished; and we accordingly conclude that a body penetrating into the bowels of the earth loses its gravity in proportion as it approaches the center. It is evident then that neither the intensity nor the direction of gravity is a consequence from the nature of any body, as not only its intensity is variable, but likewise its direction, which, on passing to the antipodes, becomes quite contrary."^[1]

After some further exposition of the efi'ects of gravitation, as observed in the courses of the planets, Euler indulges in some speculation on the probable nature of this influence. " But in attempting to dive into the mysteries of nature, it is of importance to know if the heavenly bodies act upon each other by impulsion or by attraction ; if a certain subtile invisible matter impels them toward each other ; or if they are [222] endowed with a secret or occult quality, by which they are mutually attracted. On this question philosophers are divided. Some are of opinion that this phenomenon is analogous to an impulsion j others maintain with Newton, and the English in general, that it consists in attraction."^[2]

"To avoid all confusion which might result from this mode of expression, it ought rather to be said that bodies move as if they mutually attracted each other. This would not decide whether the powers which act on bodies reside in the bodies themselves or out of them ; and this manner of speaking might thus suit both parties. Let us confine ourselves to the bodies which we meet with on the surface of the earth. Every one readily admits that all these would fall downward, unless they were supported. Now the question turns on the real cause of this fall. Some say that it is the earth which attracts these bodies, by an inherent power natural to it ; others that it is the tether, or some other subtile or invisible matter, which impels the body downward, so that the effect is nevertheless the same in both cases.

" This last opinion is most satisfactory to those who are fond of clear principles in philosophy, as they do not see how two bodies at a distance can act upon each other if there be nothing between them. … Let us suppose that before the creation of the world, God had created only two bodies, at a distance from each other ; that absolutely nothing existed outside of them, and that they were in a state of rest; would it be possible for the one to approach the other, or for them to have a propensity to approach? How could the one feel the other at a distance ? Whence could arise the desire of approaching? These are perplexing questions. But if you suppose that the intermediate space is filled with a subtile matter, we can comprehend at once that this matter may act upon the bodies by impelling them. The effect would be the same as if they possessed a power of mutual attraction. Now as we know that the whole space which separates the heavenly bodies is filled with a subtile matter called aether, it seems more reasonable to ascribe the mutual attraction of bodies to an action which the aether exercises upon them, though its manner of acting may be unknown to us, than to have recourse to an unintelligible property As the idea of all occult qualities is now banished from philosophy, attraction ought not to be considered in this sense."^[3]

It does not appear how so vague and inexplicable a supposition is calculated to commend itself "to those who are fond of clear principles in philosophy." In his anxiety to avoid an "occult quality " in matter, this learned writer seems quite unconscious of the fact that by investing his aether with an "unknown manner of acting," he is just as fatally "having recourse to an unintelligible property." Certainly, just as [223] "perplexing questions " are suggested by the hypothesis of aether pressure, as by the hypothesis of" an original "propensity to approach."

The speculation however, is too indeterminate to admit of precise criticism ; and is noteworthy only from the eminence of its proposer. It is a little remarkable that Euler, although in correspondence with Lesage, makes no allusion to his hypothesis.

1 ↑ Letters a une Princesse d'Allemagne, Let. 50, 30th August, 1760. This work, since so popular, was republished in England, " Letters on different subjects in Physics and Philosophy, addressed to a German Princess." Translated from the French by Henry Hunter, 2 vols. 8vo, London, 1802.

2 ↑ Letters, &c. Let. 54, 7th September, 1760.

3 ↑ Letters, &c. Let. 68, 18th October, 1760.