1857.] on the Conservation of Force. 353 



exemption, amongst the forms of force as regards the prhieiple 

 of conservation ; but simply that being, as far as we perceive, incon- 

 vertible in its nature and unchangeable in its manifestation, 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 indestructi- 

 bility of matter, or the invariability of gravity, I think that no par- 

 ticular 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 hypotheses, whether partial or general, regard- 

 ing the actions of matter. 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 adverse 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 themselves : 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 investiga- 

 tion, and continually increase in force. For instance, time is 

 growing up daily into importance as an element in the exercise of 

 force. 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, acting 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 in 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, there- 

 fore, not metaphysical to expect an effect in every case, or to 

 endeavour to discover its existence and determine its nature. So 

 in regard to the principle 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 hypothesis, whether of 



