OF NATURAL PHILOSOPHY. 193 



(204.) Again, the agents employed by nature to act 

 on material structures are invisible, and only to be 

 traced by the effects they produce. Heat dilates 

 matter with an irresistible force ; but what heat is, 

 remains yet a problem. A current of electricity 

 passing along a wire moves a magnetized needle at a 

 distance; but except from this effect we perceive no 

 difference between the condition of the wire when it 

 conveys and when it does not convey the stream : 

 and we apply the terms current, or stream, to the 

 electricity only because in some of its relations 

 it reminds us of something we have observed in a 

 stream of air or water. In like manner we see 

 that the moon circulates about the earth; and be- 

 cause we believe it to be a solid mass, and have 

 never seen one solid substance revolve round an- 

 other within our reach to handle and examine 

 unless retained by a force or united by a tie, we 

 conclude that there is a force, and a mode of con- 

 nection, between the moon and the earth ; though, 

 what that mode can be, we have no conception, 

 nor can imagine how such a force can be exerted 

 at a distance, and with empty space, or at most an 

 invisible fluid, between. (See $ 148.) 



(205.) Yet are we not to despair, since we 

 see regular and beautiful results brought about in 

 human works by means which nobody would, at 

 first sight, think could have any thing to do with 

 them. A sheet of blank paper is placed upon a 

 frame, and shoved forwards, and after winding its 

 way successively over and under half a dozen 

 rollers, and performing many other strange evo- 

 lutions, comes out printed on both sides. And, 



