On Force. 5 



and forward motion of particles analogous to the vibrations 

 of a pianoforte- wire, and to students, nay, even to expert 

 physicists, it is doubtless a great assistance to have the 

 hypothesis stated in that concrete and specific form. But 

 the truth of the undulatory theory is only established by 

 the agreement of its results with those of experiments, and 

 the same results could be obtained from a much more 

 general hypothesis than that usually made. It is only 

 necessary to suppose that, as Clerk Maxwell says (Elec- 

 tricity and Magnetism, Vol. II., p. 407), the disturbance 

 which constitutes light is of the nature of a vector (i.e., a 

 quantity having both magnitude and direction) perpen- 

 dicular to the ray; and all the beautiful theorems whose 

 truth has been so abundantly confirmed by experiment 

 and observation, could still be deduced if we supposed that 

 the vector disturbance is a strain, a rotation, a magnetisa- 

 tion, or electrification of particles, instead of supposing the 

 particles to have motions of translation. 



Still it would be inconvenient, if not impossible, especially 

 for purposes of instruction, to abandon the ordinary specific 

 hypothesis. In the same manner should the hypothesis of 

 forces exerted by inanimate bodies be maintained, as though 

 not necessarily true, still very convenient, and invariably 

 leading to true results. It is often said that if all calculated 

 results of an hypothesis agree with experiment, that hypo- 

 thesis must itself be true. The statement is not correct. 

 The most that we are warranted in believing is that all 

 other calculated results will also be found to be experi- 

 mentally true, and this is especially the case when the 

 hypothesis is one like that of Forces, which from its very 

 nature cannot and could not under any conceivable circum- 

 stances be directly subjected to an experimental test. Surely 

 it is more hopeless to attempt to verify the existence of 

 the earth's attraction than it is to endeavour to see the 

 vibrations of the ether. 



Professor Tait, in a lecture delivered before the British 

 Association last year, has attacked the existence of Force in 

 a different manner; and although I agree so far with his 

 conclusions as to believe that the existence of material 

 forces is not and cannot be proved, I do not believe the 

 reasoning by which he arrives at that conclusion is valid. 

 He not only believes that Force is proved not to have real 

 objective existence, but that that peculiar and abstruse 



