288 THE PRINCIPLES OF SCIENCE. 



countenance is now given to the notion by the somewhat 

 similar rate of propagation of nerve pulses and sound- 

 waves in soft bodies. But the phenomena of memory are 

 far more difficult to reduce to any material mechanism, 

 and I know of no material analogy but the interesting 

 one suggested by Hooke c , who likens memory to ' those 

 bells or vases which Vitruvius mentions to be placed in 

 the ancient theatre, which did receive and return the 

 sound more vigorous and strong; or like the unison- 

 toned strings, bells, or glasses, which receive impressions 

 from sounds without, and retain the impressions for some 

 time, answering the tone by the same tone of their own.' 



Analogy in the Mathematical Sciences. 



Whoever wishes to acquire a deep acquaintance witli 

 the constitution of Nature must observe that there are 

 deep analogies which connect whole branches of science in 

 a parallel manner, and enable us to infer of one class of 

 phenomena what we know of the other. It has thus 

 happened on several occasions that the discovery of 

 an unsuspected analogy between two hitherto distinct 

 branches of knowledge has been the starting-point for a 

 rapid course of discovery. The truths readily observed 

 in the one may be of a different character from those 

 which present themselves in the other. The analogy, 

 when once pointed out, leads us easily to discover regions 

 of one science yet undeveloped, but to which the key is 

 furnished by the corresponding truths in the other 

 science. An interchange of aid most wonderful in its 

 results may thus take place, and at the same time the 

 mind rises to a higher generalization, and a more com- 

 prehensive view of mind and nature. 



c ' Posthumous "Works/ p. 141. 



