178 ANNUAL OF SCIENTIFIC DISCOVERY. 



or can altogether disappear, would be to admit that matter could be uncre- 

 ated; for we know matter only by its forces; and though one of these is 

 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 principle of conservation, out simply that being, as far as 

 we perceive, inconvertible in its nature and unchangeable in its manifesta- 

 tion, 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 lai'ge and sure as that of the indestructibility of matter, or the 

 invariability of gravity, I think that no particular idea of force has aright to 

 unlimited or unqualified acceptance that docs 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 offeree ought to be admitted as a physical principle in 

 all our hypotheses, whether partial or general, regarding the actions of mat- 

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

 verse to the easy and unconsidered admission of one supposition upon 

 another, suggested as they often are by very imperfect induction from a 

 small number of facts, or by a very imperfect observation of the facts them- 

 selves; but, on the other hand, I think the philosopher 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 clement in the ex- 

 ercise offeree. The earth moves in its orbit in time ; the crust of the earth 

 moves in time ; light moves in time ; an electro-magnet requires time for 

 its charge by an electric current ; to inquire, therefore, whether power, act- 

 ing either at sensible or insensible distances, always acts in time, is not to be 

 metaphysical ; if it acts in time and across space, it must act by physical 

 lines of force ; and our view of the nature of the force may be affected to the 

 extremest degree by the conclusions which experiment and observation on 

 time may supply ; being, perhaps, finally determinable only by them. To 

 inquire after the possible time in which gravitating, magnetic, or electric 

 force is exerted, is no more metaphysical than to mark the times of the hands 

 of a clock in their progress ; or that of the temple of Serapis and its ascents 

 and descents ; or the periods of the occultations of Jupiter's satellites ; or 

 that in which the light from them comes to the earth. Again, in some of 

 the known cases of action in time, something happens whilst the time, is 

 passing which did not happen before, and does not continue after ; it is, 

 therefore, not metaphysical to expect an effect in every case, or to endeavor 

 to discover its existence and determine its nature. So in regard to the prin- 

 ciple of the conservation of force ; I do not think that to admit it, and its 

 consequences, whatever they may be, is to be metaphysical ; on the contrary, 

 if that word have any application to physics, then I think that any hypothe- 

 sis, whether of heat, or electricity, or gravitation, or any other form of force, 

 which either willingly or unwillingly dispenses with the principle of conserv- 



