100 WRITINGS OP JOSEPH HENRY. [1855- 



is evident that at the moment the rim of the bell is going 

 from the spectator, a tendency to a vacuum would be pro- 

 duced, and the atoms of the first layer of air will follow the 

 metal by their elasticity, thus producing a rarefaction into 

 which the atoms of the second layer of air will rush ; and 

 this will advance from layer to la3'er until it reaches the ear 

 of the observer. But before it has got far on its way, the 

 side of the bell will return, and will condense the air in con- 

 tact with it, and send a positive impulse in the same direc- 

 tion with the first. These two impulses, travelling with equal 

 velocities, and the one immediately succeeding the other, 

 form an undulation. 



The effect may be strikingly illustrated by water in a long 

 trough. If a small block of wood of the width of the trough 

 be suddenly drawn out of the liquid at one end of the trough 

 the water in immediate contact with the block will flow in 

 to fill the vacuum ; the water next will flow into the space 

 thus left, and so on, a hollow or negative wave will be prop- 

 agated from one end of the trough to the other. If the same 

 block be suddenly thrust down into the water, the effect will 

 be as if a quantity of water had been suddenly added. The 

 liquid will rise at the side of the block, and in its fall 

 another wave will be elevated outside of it, and so on con- 

 tinually, a positive wave or one of elevation, will be trans- 

 mitted to the farther extremity of the reservoir. 



If the two motions of the block be made, one immediately 

 succeeding the other, a compound wave or an undulation 

 will be the result. The transfer in this case is again that 

 of form and not of substance. The atoms of water remain 

 in place, as will be evident by placing bits of wood on the 

 surface; they will rise and fall, but will not advance as the 

 wave passes. This is an illustration of an undulation, but 

 not an exact representation of a sound wave, which consists 

 in a slightly alternate backward and forward motion of each 

 particle between the bell and the observer. 



An undulation of sound therefore consists of two parts — 

 a condensed and a rarefied part; and hence when two series 

 of undulations of the same wave length follow each other Jit 



