1024 Dynamic Theory. 



heavy matter. And further it is proved that this sort of motion cannot 

 be taken on by the uncondensed ether which pervades space, and whose 

 undulations cause light and heat, by the fact that sound cannot cross a 

 vacuum. There being no ponderable matter in the vacuum, there is no 

 condensed ether, consequently no pulse. 



It is said that the velocity of sound is increased by the elasticity of its medium, and de- 

 creased by its density. In gases the velocity increases with temperature. In general 

 terms the velocity in gases is directly as the square root of the pressure they are under, 

 and inversely as the square root of their specific gravity. The following are velocities of 

 sound in feet per second, in the substances named: lead, 4,030; gold, 5,000; copper, 

 11,000; iron, 16,822; cast steel, 10,354; alcohol (at 20 centigrade), 4,218 feet; ether (0c), 

 3,801 ; sea water (20 c), 4,768 ; fresh water (15 c), 4,714 ; oxygen (0 c), 1,040 ; carbonic 

 acid, 858; hydrogen, 4,164; air, 1090. 



Now the question arises, is the movement of the pulse through the in- 

 terstitial ether, invariably accompanied by a corresponding synchronous 

 vibration of the ponderable body? Is it not conceivable that the vibra- 

 tory pulsations of the enclosed ether may in some bodies outrun the 

 capacity for vibration of the body enclosing it? 



It is stated that the velocity of sound in a steel rail is fifteen times 

 as great as in the air, so that at a point 1,090 feet from the place where 

 a sound originates, it would be heard 225 times as loud by way of the 

 rail as by way of the air. The physical basis of loudness is amplitude, 

 and amplitude is the fulness or wideness, or in the case of sound we 

 may say the longness of the movement of the matter engaged in pro- 

 ducing a wave. The length of the wave of the small octave C, having 

 132 vibrations per second, is in air 8^- feet and its amplitude is the dis- 

 tance which the further end is driven away from the sounding body at 

 each pulsation. That is, a ray or spoke of air so long, is driven end- 

 wise a certain distance, which distance constitutes its amplitude. Now 

 in steel the length of this wave or spoke, for the same sound, is nearly 

 124 feet. The amplitude in the example given above is 225 times 

 greater than in the air. Are we to understand that the molecules of 

 124 feet of the steel rails are driven endwise against each other, so that 

 the end one in each wave moves 225 times as far as the molecule of air 

 at the end of an air wave, when both are started by the same impulse? 

 It does not seem reasonable. Again, a sound just loud enough to be 

 conveyed by the air a distance of 1,090 feet, or about one-fifth of a 

 mile, will be conveyed by the steel rail about three and one-tenth miles. 

 Now when we are told that under the impulse of the sonorous stroke all 

 the air is vibrated to a distance of one-fifth of a mile only, while all the 

 particles of steel in the conductor for a distance of three and one-tenth 

 miles are vibrated by the same stroke, it does not look reasonable. 

 Again, one may hear the chirp of a cricket in the next room, although 

 separated from it by a brick wall a foot thick. Does the vibration com- 

 municated to the air by the cricket jar the bricks and mortar of the 

 masonry? Numerous experiments and observations do indeed show 



