184 CORRELATION OF PHYSICAL FORCES. 



The probability is, that, if not all, the greater number of 

 physical phenomena are correlative, and that, without a 

 duality of conception, the mind cannot form an idea of them : 

 thus motion cannot be perceived or probably imagined with- 

 out parallax or relative change of position. The world was 

 believed fixed, until by comparison with the celestial bodies, 

 it was found to change its place with regard to them : had 

 there been no perceptible matter external to the world, we 

 should never have discovered its motion. In sailing along a 

 river, the stationary vessels and objects on the banks seem 

 to move past the observer : if at last he arrives at the convic- 

 tion that he is moving, and not these objects, it is by correct- 

 ing his senses by reflection derived from a more extensive 

 previous use of them : even then he can only form a notion 

 of the motion of the vessel he is in, by its change of position 

 with regard to the objects it passes that is, provided his 

 body partakes of the motion of the vessel, which it only does 

 when its course is perfectly smooth, otherwise the. relative 

 change of position of the different parts of the body and the 

 vessel inform him of its alternating, though not of its pro- 

 gressive movement. So in all physical phenomena, the effects 

 produced by motion are all in proportion to the relative mo- 

 tion : thus, whether the rubber of an electrical machine be 

 stationary, and the cylinder mobile, or the rubber mobile and 

 the cylinder stationary, or both mobile in different directions, 

 or in the same direction with different degrees of velocity, 

 the electrical effects are, cceteris paribus, precisely the same, 

 provided the relative motion is the same, and so, without ex- 

 ception, of all other phenomena. The question of whether 

 there can be absolute motion, or, indeed, any absolute isolated 

 force, is purely the metaphysical question of idealism or real- 

 ism a question for our purpose of little import ; sufficient 

 for the purely physical inquirer, the maxim ' de non apparenti- 

 bus et non existentibus eadem est ratio. 9 



The sense I have attached to the word correlation, in 



