443 Prof. Challis on the Force of Gi-avity. 



generating lines are finite lines of equal length, and the curved 

 contours shown in the figures are consequently the spherical 

 curves which are the intersections of the cones by concentric 

 spheres. The figures are intended to be looked at with the 

 glasses of a Reeves's book stereoscope. 

 2 Stone Buildings, W.C, 

 September 26, 1859. 



LXIX. A Theory of the Force of Gi-avity. 

 By Professor Challis*. 



THE undertaking on which I have entered, that of inves- 

 tigating mathematically the laws of the physical forces, fails 

 altogether if the investigation does not embrace the laws of gra- 

 vity ; for it is impossible that this force can be put in a different 

 category from that of the others. I propose, therefore, to inquire, 

 since gra\dty is an attractive force, how far its laws and properties, 

 as known by experience and the results of calculation, can be 

 accounted for by the Mathematical Theory of Attractive Forces 

 which I have given in the Number of the Philosophical Maga- 

 zine for November. But first a few preliminary remarks must 

 be made. 



The actio in distans has been so long and so extensively re- 

 garded as an ultimate principle, and not as a temporary hypo- 

 thesis admitting eventually of explanation, that it requires some 

 degree of moral courage to maintain a different theory. Science, 

 in my opinion, is much indebted to Professor Faraday for having 

 recently directed attention to the opposite views entertained by 

 Newton on this point, and for giving expression to analogous 

 ideas of his own. (See the Lecture on the Conservation of Force 

 in the Phil. Mag. for April 1857, vol. xiii. p. 332.) It is quite 

 true, as Professor Faraday remarks, that the mathematician is 

 not in a better situation than the experimentalist for originating 

 conceptions of the nature of the physical forces ; and at least 

 it will be conceded that the impressions, or connctions, on this 

 subject of a philosopher whose experimental researches have 

 made him famihar with the action and effects of physical forces 

 in a great variety of forms, are entitled to much consideration. 

 At the same time it is true that all such antecedent concep- 

 tions must be tested by the comparison with facts of deductions 

 drawn from them by mathematics, and only after being so tested 

 can they be added to the stock of common knowledge. 



The views pvit forth by Faraday have been contested by Pro- 

 fessor Ernest Briicke of Vienna (Phil. Mag. for February 1858, 

 p. 81), who endeavours by a priori reasoning to connect i\\e, actio 

 in distans with conservation of vis viva, as if the latter were an 

 * Communicateil bv the Author. 



