402 Dr. Faraday on the Physical Character of 



the indications given by a magnetic needle, and in numerous 

 cases with great and peculiar advantages. The definition then 

 given had no reference to the physical nature of the force at the 

 place of action, and will apply with equal accuracy whatever 

 that may be ; and this being very thoroughly understood, I am 

 now about to leave the strict line of reasoning for a time, and 

 enter upon a few speculations respecting the physical character 

 of the lines of force, and the manner in which they may be sup- 

 posed to be continued through space. We are obliged to enter 

 into such speculations with regard to numerous natural powers, 

 and, indeed, that of gravity is the only instance where they are 

 apparently shut out. 



3244;. It is not to be supposed for a moment that speculations 

 of this kind are useless, or necessarily hurtful, in natural philo- 

 sophy. They should ever be held as doubtful, and liable to 

 error and to change ; but they are wonderful aids in the hands 

 of the experimentalist and mathematician ; for not only are they 

 useful in rendering the vague idea more clear for the time, giving 

 it something like a definite shape, that it may be submitted to 

 experiment and calculation ; but they lead on, by deduction and 

 correction, to the discovery of new phenomena, and so cause an 

 increase and advance of real physical truth, which, unlike the 

 hypothesis that led to it, becomes fundamental knowledge not 

 subject to change. Who is not aware, of the remarkable pro- 

 gress in the development of the nature of light and radiation in 

 modern times, and the extent to which that progress has been 

 aided by the hypotheses both of emission and undulation ? Such 

 considerations form my excuse for entering now and then upon 

 speculations ; but though I value them highly when cautiously 

 advanced, I consider it as an essential character of a sound mind 

 to hold them in doubt; scarcely giving them the character of 

 opinions, but esteeming them merely as probabilities and pos- 

 sibilities, and making a very broad distinction between them 

 and the facts and laws of nature. 



3245. In the numerous cases of force acting at a distance, 

 the philosopher has gradually learned that it is by no means 

 sufficient to rest satisfied with the mere fact, and has therefore 

 directed his attention to the manner in which the force is trans- 

 mitted across the intervening space ; and even when he can learn 

 nothing sure of the manner, he is still able to make clear distinc- 

 tions in different cases, by what may be called the affections of 

 the lines of power; and thus, by these and other means, to make 

 distinctions in nature between the lines of force of different 

 kinds, or exertions, of power as compared with each other, and 

 therefore between the powers to which they belong. In the 

 action of gravity, for instance, the line of force is a straight line 



