KINETIC THEORIES OF GRAVITATION. 235 



experience, as a question of fact, the keenest of experimental investi- 

 gators, Faraday himself, has been able to detect no flaw, the utter 

 falsity of the inference may be taken as conclusive against the premiss. 

 Gravity is thereby proved to be a static tension^ — incessant, inconverti- 

 ble, inexhaustible; as affirmed by our fifth and sixth propositions. 

 Whatever a iiriori conceptions may be indulged therefore, as to the 

 natural fitness of a central force having the same tension at all dis- 

 tances, it has been definitely established by two centuries of continu- 

 ous and irreversible demonstration, that gravity is not such a force. 

 And this announcement is the subject of our third proposition. 



" It will not be imagined for a moment," says Faraday, " that I am 

 opposed to what may be called the law of gravitative action; that is, the 

 law by which all the known effects of gravity are governed. What I 

 am considering is the definition of the force of gravitation. . . . 

 That the totality of a force can be employed according to that law I do 

 not believe, either in relation to gravitation, or electricity, or magnetism, 

 or any other supposed form of power." * But the most refined and varied 

 observations (even when conducted by a Faraday) have failed to detect 

 any such supposed residuum of effect, and have substantiated as one of 

 the largest results of our present knowledge the received formula as 

 expressing the "totality" of the force recognized as gravity. Our 

 "beliefs" should always be based upon, and conform to, the observed 

 order of nature. "The safest course appears to be to assume as little 

 as possible." 



Faraday thus sums up his own impressions: "For my own part, many 

 considerations urge my mind toward the idea of a cause of gravity which 

 is not resident in the particles of matter merely, but constantly in them 

 and all space." (p. 231.) " I would much rather incline to believe that 

 bodies affecting each other by gravitation act by lines of force of definite 

 amount, or by an rether pervading all parts of space, than admit that the- 

 conservation of force could be dispensed with." (238.) Fortunately, 

 the alternative presented possesses no relation of its terms. The un- 

 qualified assertion of " conservation" has no bearing whatever on either 

 "lines of force" or the supposed action of " an aither;" and a choice is 

 therefore quite unnecessary.! 



On no subject, perhaps, have the distinguished author's ideas been 

 more vague and intangible than on the favorite one of " lines of force." 

 After exhibiting the familiar magnetic curves or chains of iron-filings as 

 a typical phenomenon, he says : " The term line of force, as defined above, 

 is restricted to mean no more than the condition of the force in a given 

 place as to strength and direction; and not to include any idea of the 

 nature of the physical cause of the phenomena. At the same time, if 



^ Loco chat., p. 233. 



t An excellent review and criticism of Professor Faraday's Memoir on Gravitation, 

 by Professor Briicke, of Vienna, was published in the L. E.D., Phil. Mag., 185$, vol. xv, 

 p. 81. 



