SCIENTIFIC USE OF THE IMAGINATION. 1Q5 



body plunged in the water, as the wavelets reach it in 

 succession. But a finer motion is at the same time set 

 up and propagated. If the head and ears be immersed 

 in the water, as in an experiment of Franklin's, the 

 tick of the drop is heard. Now, this sonorous impulse 

 is propagated, not at the rate of a foot, but at the rate 

 of 4,700 feet a second. In this case it is not the 

 gravity but the elasticity of the water that comes into 

 play. Every liquid particle pushed against its neigh- 

 bour delivers up its motion with extreme rapidity, 

 and the pulse is propagated as a thrill. The incom- 

 pressibility of water, as illustrated by the famous Flor- 

 entine experiment, is a measure of its elasticity; and 

 to the possession of this property, in so high a degree, 

 the rapid transmission of a sound-pulse through water 

 is to be ascribed. 



But water, as you know, is not necessary to the 

 conduction of sound; air is its most common vehicle. 

 And you know that when the air possesses the particu- 

 lar density and elasticity corresponding to the tem- 

 perature of freezing water, the velocity of sound in it 

 is 1,090 feet a second. It is almost exactly one-fourth 

 of the velocity in water; the reason being that though 

 the greater weight of the water tends to diminish the 

 velocity, the enormous molecular elasticity cf the 

 liquid far more than atones for the disadvantage due 

 to weight. By various contrivances we can compel the 

 vibrations of the air to declare themselves; we know 

 the length and frequency of the sonorous waves, and 

 we have also obtained great mastery over the various 

 methods by which the air is thrown into vibration. 

 We know the phenomena and laws of vibrating rods, of 

 organ-pipes, strings, membranes, plates, and bells. We 

 can abolish one sound by another. We know the 

 physical meaning of music and noise, of harmony and 



