166 CORRELATION OF PHYSICAL FORCES. 



used has, I think, justified me. Its use has, in my judgment, 

 been carried too far in applying it to subjects quite beyond 

 its fair meaning. There are many facts, one of which cannot 

 take place without involving the other ; one arm of a lever 

 cannot be depressed without the other being elevated the 

 finger cannot press the table without the table pressing the 

 finger. A body cannot be heated without another being 

 cooled, or some other force being exhausted in an equivalent 

 ratio to the production of heat ; a body cannot be positively 

 electrified without some other body being negatively electrified, 

 &c. To such cases the term correlation may be usefully 

 applied, but hardly to adaptations of structure, &c. 



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

 physical phenomena are, in one sense, correlative, and that, 

 without a duality of conception, the mind cannot form an 

 idea of them : thus, motion cannot be perceived or probably 

 imagined without 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 conviction that he is moving, and not these objects, it 

 is by correcting 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 fully of its 

 progressive movement. So in all physical phenomena, the 

 effects produced by motion are all in proportion to the relative 

 motion : 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, cczteris paribiis, precisely the same, pro- 



