INTRODUCTION TO PNEUMATICS. Ixxi 



afternoon, to restore the equilibrium which had been disturbed by reflec- 

 tions from the heated surface of the shore during the earlier part of the 

 day ; and about midnight, when the earth is cooled and the air condensed, 

 it flows back towards the sea. 



The air, being a gravitating fluid, is affected by the attraction of the 

 moon and the sun, in the same manner as the water, and must there- 

 fore have tides. These tides, however, are of no practical interest or 

 importance. 



We have considered the effects produced by the wide and extended 

 agitation of the air ; but there is another kind of agitation of which the 

 air is v susceptible a sort of vibratory tremulous motion, which, striking 

 on the drum of the 'ear, produces Sound. Sonorous bodies, such as bells, 

 musical instruments, &c., are merely the agents by means of which that 

 peculiar species of motion is communicated to the air. A bell runs; in 

 vacuo under the air-pump gives no sound. 



Air, though by far the most common, is not the only vehicle of sound. 

 Liquids are capable of conveying the vibratory motion of a sonorous body 

 to the organ of hearing; for sound can be heard under water. Solid bodies 

 also convey sound, as you may be convinced by a very simple experiment. 

 If a string be fastened, round a poker, by the middle; the poker raised 

 from the ground by the two ends of the string, and one end being held to 

 each ear, if the poker be then struck with a key, the sound will be con- 

 veyed to the ear by means of the strings in a much more perfect manner 

 than if it had no other vehicle than the air. 



Bodies are called sonorous which produce clear, distinct, regular, and 

 durable sounds such as a bell, a drum, musical strings, wind instru- 

 ments, &c. They owe this property to their elasticity; for an elastic 

 body, after having been struck, not only returns to its former situation, 

 but, having acquired momentum by its velocity, like the pendulum, it 

 springs out on the^ opposite side. If the string A B (Jig. 4), which is 



[Fig. 4. 



made fast at both ends, be drawn on one side to C, it will not only 

 return to its original position, but proceed onwards to D. This is the 

 first vibration; at its t termination, the string, being stretched into the 

 position A D B, will again tend to return to its natural state A B, and 

 will, therefore, return to that line and pass on beyond it to E, and thence 

 back again to F : then in the same manner to G and H ; the resistance 

 of the air continually destroying some of the motion, so that the extreme 

 points, EFG H, are continually nearer to the line AB, until the whole 

 motion is destroyed, and the string comes to rest in the position A B. 



The tremulous motion given to the air by the vibration of a sonorous 

 body is very similar to the motion communicated to smooth water when a 

 stone is thrown into it. This first produces a small circular wave round 

 the spot in which the stone falls : the wave spreads, and gradually com- 

 municates its motion to the adjacent waters, producing^similar waves to 



