Sound. 379 



not be heard. The molecular vibrations are there of course. The bell 

 changes its shape, elongating its diameter alternately in directions at 

 right angles to each other, but no air being there to take up the vibra- 

 tory movement it is not conveyed to the ear and does not become sound. 

 The effect of the vibration in this case would appear as heat in the in- 

 creased temperature of the bell and this increased temperature might be 

 carried across that vacuum. A small amount of the vibratory motion 

 might be carried up the string and so out to the air as a faint sound, 

 but only enough to prove that the absence of air inside is, as far as it is 

 concerned, a perfect bar to such vibrations as produce sound. 



It is important to understand clearly the nature of vibration in order to 

 comprehend how it is that energy transfers itself from one body to another 

 without the transfer of the bodies themselves. It is difficult to realize 

 this without special attention. Let us suppose a clothesline to be 

 stretched from one post to another. If the line be agitated at one end 

 by a slight transverse blow, we shall see a wave travel along the line 

 from that end to the other. It is plain enough that it is the movement 

 that goes end- wise and not the line as a whole. Each small section of 

 the line makes a little excursion first to one side then to the other, and 

 it is these excursions taking place in one section after another in pro- 

 gressive succession that constitute the movement of the wave. As the 

 force of the pull is communicated to the first section of the line in caus- 

 ing its transverse movement, so as the second section is started in motion 

 hy the first, it receives the energy of the first, transfers it to the third, 

 &c., so the energy goes with the wave from one end to the other. It 

 goes with it, neither before it nor behind it. By a properly contrived 

 connection the energy and the vibration might be transferred at the end 

 to another line, or to any other sort of elastic body, as a bell for ex- 

 ample. Notwithstanding the line as a whole cannot be said to move 

 endwise, yet a little further calculation will show that each particle of 

 it does not only make a transverse excursion, but also a small endwise 

 excursion forward and back. As each wave is formed, the section of the 

 line concerned momently occupies the position of a diagonal or hypot- 

 enuse in relation to the general direction of the line, and, therefore, for 

 the time, acquires additional length at the expense of the elasticity of 

 the line, or what is equivalent in this case, at the expense of its slack. 

 The same must be true of all sorts of vibration. If a pebble be dropped 

 into still water the progressive series of concentric waves that results, in 

 effect stretches the surface of the water by a change in the relative posi- 

 tion of its molecules, because measured over the corrugations the super- 

 ficial area is greater than while the water is level. In forming waves, 

 therefore, the particles of the body in motion must move forward and 

 back. This being the case a body not elastic could not take a vibratory 



