ACTION OF FORCES IN TIME. 361 



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 gravita- 

 ting, magnetic, or electric force is exerted, is no more meta- 

 physical 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 en- 

 deavour 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 heat, or electricity, or gravitation, or 

 any other form of force, which either willingly or unwillingly 

 dispenses with the principle of conservation, is more liable to 

 the charge than those which, by including it, become so far 

 more strict and precise. 



Supposing that the truth of the principle of the conserva- 

 tion of force is assented to, I come to its uses. No hypothesis 

 should be admitted, nor any assertion of a fact credited, that 

 denies the principle. No view should be inconsistent or in- 

 compatible with it. Many of our hypotheses in the present 

 state of- science may not comprehend it, and may be unable 

 to suggest its consequences ; but none should oppose or con- 

 tradict it. 



If the principle be admitted, we perceive at once that a 

 theory or definition, though it may not contradict the princi- 

 ple, cannot be accepted as sufficient or complete unless the 

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