79 2 THE POPULAR SCIENCE MONTHLY. 



pothesis 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 de- 

 veloped. 



The difficulty in accepting the fact of gravity seems to be a meta- 

 physical one, though even the metaphysicians have not held that con- 

 ceivability 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 mathe- 

 matics. The sentiment, however, such as it is, has led to many inge- 

 nious 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 mathemati- 

 cal concepts ; being an ideal abstraction as much so as a surface, or 

 a line, or four-dimensioned space invented for the purpose of investi- 

 gating problems in hydrodynamics. A homogeneous, incompressible, 

 continuous, perfectly mobile but not miscible substance is an impos- 

 sible 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 char- 

 acteristic 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 ex- 

 plain away the weight of matter, especially since no such invention 

 has been so perfected as to prove particularly successful for the pur- 



