360 THE CONSERVATION OF FOKCE. 



most commonly referred to, namely, gravity, to prove its 

 presence, it is not because gravity has any pretension, or any 

 exemption, amongst the forms of force as regards the princi- 

 ple of conservation, but simply that being, as far as we per- 

 ceive, inconvertible in its nature and unchangeable in its man- 

 ifestation, it offers an unchanging test of the matter which we 

 recognize by it. 



Agreeing with those who admit the conservation of force 

 to be a principle in physics, as large and sure as that of the 

 indestructibility of matter, or the invariability of gravity, I 

 think that no particular idea of force has a right to unlimited 

 or unqualified acceptance that does not include assent to it ; 

 and also, to definite amount and definite disposition of the 

 force, either in one effect or another, for these are necessary 

 consequences ; therefore I urge, that the conservation of force 

 ought to be admitted as a physical principle in all our hypoth- 

 eses, whether partial or general, regarding the actions of mat- 

 ter. I have had doubts in my own mind whether the consid- 

 erations I am about to advance are not rather metaphysical 

 than physical. I am unable to define what is metaphysical in 

 physical science ; and am exceedingly adverse to the easy and 

 unconsidered admission of one supposition upon another, sug- 

 gested as they often are by very imperfect induction from a 

 small number of facts, or by a very imperfect observation of 

 the facts themselves ; but, on the other hand, I think the phi- 

 losopher may be bold in his application of principles which 

 have been developed by close inquiry, have stood through 

 much investigation, and continually increase in force. For 

 instance, time is growing up daily into importance as an ele- 

 ment in the exercise of force. The earth moves in its orbit 

 in time ; the crust of the earth moves in time ; light movies in 

 time ; an electro-magnet requires time for its charge by an 

 electric current ; to inquire, therefore, whether power, acting 

 either at sensible or insensible distances, always acts in time, 

 is not to be metaphysical ; if it acts in time and across space, 



