18 UNIVERSAL GRAVITATION. 



from the earth's centre. This step accomplished, it was 

 not for such a mind as Newton's there to rest. He col- 

 lected " from mathematical reasoning, unexceptionably 

 demonstrated, that all bodies which are moved in any 

 curve line described in a plane, and which, by a radius 

 drawn to a point, either quiescent or moved in any 

 manner, describe areas round that point proportional to 

 the times, are urged by forces directed to that point. 

 And since it is agreed among astronomers that the pri- 

 mary planets describe about the sun, and the secondary 

 planets describe about their primaries, areas propor- 

 tional to their times, it follows, that the force by which 

 they are continually deflected from the rectilinear tan- 

 gents are made to revolve in curvilinear orbits, is 

 directed towards bodies placed in the centres of those 

 orbits. This force, from whatever cause it may be sup- 

 posed to arise, he thought might not improperly be 

 called centripetal with respect to the revolving body 

 attractive with respect to the central body." But he 

 uses great caution to prevent a misinterpretation of his 

 meaning, which I mention here, because I have heard 

 very confident arguments against his philosophy, founded 

 on a mistake as to this point. 



" What I call attraction" he suggests, " may be per- 

 formed by impulse, or by some other means unknown 

 to me. I use that word here to signify in general any 

 force by which bodies tend towards one another, what- 

 ever be the cause ; for we must learn from the pheno- 

 mena of nature, what bodies attract one another, and 

 what are the laws and properties of the attraction, 

 before we inquire the cause by which the attraction is 

 performed." 



How immense and fertile the region of inquiry which 



