6 INDUCTION. 



Differences of duration, or of velocity, are evidently differences 

 in degree only ; and differences of direction in space, which 

 alone has any semblance of heing a distinction in kind, entirely 

 disappear (so far as our sensations are concerned) by a change 

 in our own position ; indeed the very same motion appears to 

 lis, according to our position, to take place in every variety of 

 direction, and motions in every different direction to take place 

 in the same. And again, motion in a straight line and in a 

 curve are no otherwise distinct than that the one is motion 

 continuing in the same direction, the other is motion which at 

 each instant changes its direction. There is, therefore, accord- 

 ing to the principles I have stated, no absurdity in supposing 

 that all motion may be produced in one and the same way ; 

 by the same kind of cause. Accordingly, the greatest achieve- 

 ments in physical science have consisted in resolving one 

 observed law of the production of motion into the laws of other 

 known modes of production, or the laws of several such modes 

 into one more general mode ; as when the fall of bodies to the 

 earth, and the motions of the planets, were brought under the 

 one law of the mutual attraction of all particles of matter; 

 when the motions said to be produced by magnetism were 

 shown to be produced by electricity ; when the motions of 

 fluids in a lateral direction, or even contrary to the direction of 

 gravity, were shown to be produced by gravity ; and the like. 

 There is an abundance of distinct causes of motion still unre- 

 solved into one another ; gravitation, heat, electricity, chemical 

 action, nervous action, and so forth ; but whether the efforts 

 of the present generation of savans to resolve all these dif- 

 ferent modes of production into one, are ultimately successful 

 or not, the attempt so to resolve them is perfectly legitimate. 

 For though these various causes produce, in other respects, 

 sensations intrinsically different, and are not, therefore, capable 

 of being resolved into one another, yet in so far as they all 

 produce motion, it is quite possible that the immediate ante- 

 cedent of the motion may in all these different cases be the 

 same ; nor is it impossible that these various agencies them- 

 selves may, as the new doctrines assert, all of them have for 

 their own immediate antecedent, modes of molecular motion. 



