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792 of Lucretius, especially to get rid of the (to him) inconceivable notion of gravity. Those kinematists who follow Le Sage do so with the same avowed motive. Another school, with the same view, have revived the continuous notion of matter; out of which they have constructed an atom which has permanence and elasticity, but no avowed occult affection except inertia. It has not been further developed.

The difficulty in accepting the fact of gravity seems to be a metaphysical one, though even the metaphysicians have not held that conceivability is a criterion of objective truth. The irrelevancy of this objection has been well stated by Mr. W. R. Browne, in his article on "Central Forces" ("London, Edinburgh, and Dublin Philosophical Magazine," January, 1883, page 40), as follows: "I am not aware that the term 'unthinkable,' which is a new one, has ever been defined. Until it has been, it is impossible to say whether action at a distance is unthinkable, or whether the fact of a conception being unthinkable is sufficient reason, or any reason, for holding it to be untrue." The many instances of unthinkable truths within our familiar knowledge will readily recur to all in illustration, as, for instance, the infinite extension of space, the infinite approach of asymptotes, the nature of interminable series, etc. In fact, all forms of absolute knowledge are unthinkable. The refusal to recognize this form of knowledge has led to much heresy in other branches of exact inquiry—even in mathematics. The sentiment, however, such as it is, has led to many ingenious and futile devices in the branch we are now considering—among others, the invention of the vortex atom, before referred to.

The vortex atom belongs, not to physics, but to purely mathematical concepts; being an ideal abstraction—as much so as a surface, or a line, or four-dimensioned space—invented for the purpose of investigating problems in hydrodynamics. A homogeneous, incompressible, continuous, perfectly mobile but not miscible substance is an impossible entity, and it would seem an inconsistent one as to mobility; and, if vortex motion can not be destroyed in it, it is equally true that no means can be devised for originating it. An occult force had to be attributed to it, after all, as mass. Helmholtz, its inventor, discussed it as a purely mathematical problem; but its British adopters, struck with the remarkable attributes deduced from the postulates, set it up as the basis of a kosmos. By a similar appreciation, when that characteristic product of British genius, a modern plow, was carried to India—the land of theosophic contemplation—its enthusiastic foreign admirers, after having been carefully shown its merits, and instructed in its use, were found to have erected it in the center of the field as a god!

But, though we have no need of the hypothesis of an ether to explain away the weight of matter, especially since no such invention has been so perfected as to prove particularly successful for the