SCIENTIFIC USE OF THE IMAGINATION 113 



lin'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 grav- 

 ity, but the elasticity of the water that comes into play. 

 Every liquid particle pushed against its neighbor delivers 

 up its motion with extreme rapidity, and the pulse is 

 propagated as a thrill. The incompressibility 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 con- 

 duction of sound; air is its most common vehicle. And 

 you know that when the air possesses the particular den- 

 sity 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 them- 

 selves ; 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 vibra- 

 tion. 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 dis- 

 cord. In short, as regards sound in general, we have a 

 very clear notion of the external physical processes which 

 correspond to our sensations. 



