KINETIC THEORIES OF GRAVITATION. 279 



Ainong the names presented in the foregoing selection from kinetic 

 theorists are several eminent in the domains of physics and mathemat- 

 ics. Their labors, animated with the zeal, the diligence, and the persist- 

 ency of purpose and conviction here displayed, might be expected to 

 enlarge notably the boundaries of our knowledge to whatever depart- 

 ment of natural law they were devoted. But when we look for any 

 actual results from these elaborated speculations, how barren is the 

 prospect. How wonderfully contrasted in character is the prime as- 

 sumption with the fruitfulness of that generalization which it aims to 

 subordinate and to comprehend. How striking its failure, not in the 

 higher role of prophecy, but in the humbler one of mere interpretation. 

 How clumsy the mechanism by which it vainly strives to accomplish its 

 results. 



But it is not simply in the negative aspects of an unsuccessful effort 

 that these varied speculations, prompted by a common sentiment and 

 motive, teach us their most suggestive lesson. Beyond the facts of con- 

 stant and of signal failure, these restless, resolute probings by the human 

 mind in all directions, serve as the cumulative bases of a new induc- 

 tion ; and by the very sharpness of the contrasts brought to view at 

 every point between the tentative conjecture and the determinate expe- 

 rience, they enforce the assurance, that whatever else the principle of 

 gravitation may be, it is not in its essence any form of motion. And 

 that gravitation is not a resultant of pressure, appears to be very clearly 

 made out by the inability of every scheme of such hypothesis, in the 

 hands of its most skilfull adherents, to give any rational account of the 

 semi-diurnal ocean-tides. 



Another lesson no less striking, is the utter worthlessness of metaphys- 

 ical axioms as a criterion of physical truth, or as a foundation for ra- 

 tional physical theory. Such propositions as "It is impossible to im- 

 agine an infinite attribute belonging to a finite entity," or " It is im- 

 possible to conceive anything to act where it is not," are not merely quite 

 irrelevant to scientific verity, but if gravely accepted as physical postu- 

 lates, prove only to be positive obstructions to scientific investigation. 

 Professor Ghallis has well said, " I do not admit that any metaphysical 

 argument can be adduced either in support of or against a physical hy- 

 pothesis, ilfefa-physics come after physics." * And Sir John Herschel, 

 some thirty years ago, laid down as a rule of sound philosophy, " Expe- 

 pience once recognized as the fountain of all our knowledge of nature } 

 it follows that in the study of nature and its laws we ought at once to 

 make up our minds to dismiss as idle prejudices, or at least suspend as 

 premature, any preconceived notion of what might or what ought to be 

 the order of nature in any proposed case, and content ourselves with 

 observing, as a plain matter of fact, what isS'i And again, long before 

 him, Newton condemned the "feigning hypothesis for explaining all 

 thiugs mechanically, and referring other causes to metaphysics. Where- 



* L. E. D. Phil. Mag., 1861, vol. xxi, p. 505. 



\ Discourse on Study of Natural Philosophy, part ii, chap, i, sec. 68. 



