Ixxii INTRODUCTION TO PNEUMATICS. 



a considerable extent. The same kind of waves are produced in the air 

 by the motion of a sonorous body ; but with this difference, that air, being 

 an elastic fluid, the motion does not consist of regularly extending 

 waves, but of vibrations composed of a motion forwards and backwards, 

 similar to those of a sonorous body. The aerial undulations also take 

 place in all directions, and are spherical. The first sphere of undulations 

 which are produced immediately around the sonorous body, by pressing 

 against the contiguous air, condenses it. The condensed air, though 

 impelled forward by the pressure, re-acts on the first set of undulations, 

 driving them back again. The second set of undulations which have 

 been put in motion, in their turn communicate their motion, and are 

 themselves driven back by re-action. Thus there is a succession of waves 

 in the air. The air is immediately put in motion by the firing of a 

 cannon ; but it requires time for the vibrations to extend to any distant 

 spot. The velocity of sound in air is computed to be at the rate of 1142 

 feet in a second. 



The direction of the wind makes less difference in the velocity of sound 

 than might be imagined. If the wind sets from us, it bears most of the 

 aerial waves away, and renders the sound fainter; but it is not very 

 considerably longer in reaching the ear than if the wind blew towards us. 

 In fact, the wind cannot possibly retard the progress of the sound, by 

 more than its own rate of motion : and as the velocity of sound is about 

 780 miles in an hour, the velocity of even a high wind bears too small a 

 proportion to it to affect very materially the rate at which sound travels. 

 The nearly uniform velocity of sound enables us to determine the distance 

 of the object whence it proceeds : as that of a vessel at sea firing a cannon, 

 or that of a thunder-cloud. If we do not hear the thunder till half a 

 minute after we see the lightning, we conclude the cloud to be at the dis- 

 tance of six miles and a half. 



An echo is produced when the aerial vibrations meet with an obstacle 

 having a hard and regular surface, such as a wall, or rock: they may thus 

 be reflected back to the ear, and produce the same sound a second time ; 

 but the sound will then appear to proceed from the object by which it 

 is reflected. If the vibrations fall perpendicularly on the obstacle, they 

 are reflected back in the same line j if obliquely, the sound returns 

 obliquely on the other side of the perpendicular, the angle of reflection 

 being equal to the angle of incidence. 



Speaking-trumpets are constructed on the principle of the reflection of 

 sound. The voice, instead of being diffused in the open air, is confined 

 within the trumpet; and the vibrations which spread and fall against the 

 sides of the instrument, are reflected according to the angle of incidence, 

 and the form of the instrument is so regulated, that the whole of the 

 vibrations are collected into a focus ; and if the ear be situated in or 

 near that spot, the sound is prodigiously increased. Fig. 5 represents the 



Fig. 5. 



speaking-trumpet; the rays, as they issue from its mouth, are distin- 

 guished by being dotted ; and they are brought to a focus at F. The 



