SUPPLEMENTARY CONSIDERATIONS. 379 



we have that is good and safe, as the steam-engine, the elec- 

 tric-telegraph, &c., witness to that principle it would require 

 a perpetual motion, a fire without heat, heat without a source, 

 action without reaction, cause with effect, or effect without a 

 cause, to displace it from its rank as a law of nature. 



During the year that has passed since the publication of 

 the foregoing views regarding gravitation, &c., I have come to 

 the knowledge of various observations upon them, some 

 adverse, others favourable : these have given me no reason to 

 change my own mode of viewing the subject ; but some of 

 them make me think that I have not stated the matter with 

 sufficient precision. The word " force" is understood by 

 many to mean simply " the tendency of a body to pass from 

 one place to another," which is equivalent, I suppose, to the 

 phrase " mechanical force ; " those who so restrain its mean- 

 ing must have found my argument very obscure. What I 

 mean by the word " force," is the cause of a physical action ; 

 the source or sources of all possible changes amongst the 

 particles or materials of the universe. 



It seems to me that the idea of the conservation of force is 

 absolutely independent of any notion we may form of the 

 nature of force or its varieties, and is as sure and may be as 

 firmly held in the mind, as if we, instead of being very 

 ignorant, understood perfectly every point about the cause of 

 force and the varied effects it can produce. There may be 

 perfectly distinct and separate causes of what are caUed 

 chemical actions, or electrical actions, or gravitating actions, 

 constituting so many forces ; but if the " conservation of 

 force " is a good and true principle, each of these forces must 

 be subject to it : none can vary in its absolute amount ; each 

 must be definite at all times, whether for a particle, or for all 

 the particles in the universe ; and the sum also of the three 



