SCIENTIFIC USE OF THE IMAGINATION. 105 



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 

 neighbour 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 

 Florentine 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 particular 

 density and elasticity corresponding to the temperature 

 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 of 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 discord. In short, 



