198 DISCOURSE ON THE STUDY 



the theory of gravitation we suppose an agent, viz. 

 force, or mechanical power, to act on any material 

 body which is placed in the presence of any other, 

 and to urge the two mutually towards each other. 

 This is a vera causa; for heavy bodies (that is, 

 all "bodies, but some more, some less,) tend to, or 

 endeavour to reach, the earth, and require the 

 exertion of force to counteract this endeavour, or 

 to keep them up. Now, that which opposes and 

 neutralizes force is force. And again, a plumb-line, 

 which, when allowed to hang freely, always hangs 

 perpendicularly ; is found to hang observably aside 

 from the perpendicular when in the neighbourhood 

 of a considerable mountain ; thereby proving that a 

 force is exerted upon it, which draws it towards the 

 mountain. Moreover, since it is a fact that the 

 moon does circulate about the earth, it must be 

 drawn towards the earth by a force ; for if there 

 were no force acting upon it, it would go on in a 

 straight line without turning aside to circulate in an 

 orbit, and would, therefore, soon go away and be lost 

 in space. This force, then, which we call the force 

 of gravity, is a real cause. 



(210.) We have next to consider the laws which 

 regulate the action of these our primary agents; 

 and these we can only arrive at in three ways : 1 st, By 

 inductive reasoning; that is, by examining all the cases 

 in which we know them to be exercised, inferring, 

 as well as circumstances will permit, its amount or 

 intensity in each particular case, and then piecing 

 together, as it were, these disjecta membra, general- 

 izing from them, and so arriving at the laws desired ; 

 2dly, By forming at once a bold hypothesis, par- 



