448 Mr. 8. T. Preston on the Mode of 



according to the principles of impact of equal masses. The 

 sphere 1 therefore returns towards the plane with its origi- 

 nal normal velocity unchanged, and receives a second similar 

 increment of velocity from the plane, which it again trans^ 

 fers, &c. The sphere 2 at once transfers to sphere 3 the in- 

 crement of velocity received from sphere 1, the sphere 2 re- 

 turning with its original normal velocity to repeat the process. 

 The same considerations apply to all the spheres ; and in this 

 *way during one forward swing of the plane A, a succession of 

 small increments of velocity are propagated in the form of a 

 wave by exchange of motion along the line of spheres (or the 

 wave consists in a peculiarity in the motion of the spheres 

 such that they move forward with a velocity somewhat greater 

 than the normal velocity, and backward with the normal ve- 

 locity), the velocity of transmission of the wave being that of 

 the spheres themselves, assuming that the diameter of the 

 spheres is small compared with their mean distance, as is 

 true of the molecules of gases. The length of this pulse 

 or half-wave evidently must depend on the time taken by 

 the plane to make one swing or semivibration ; or wave- 

 length is proportional to vibrating-period. The wave-length 

 will also evidently depend on the normal velocity of the 

 spheres. By the backward swing of the plane, to finish one 

 complete vibration, the plane A moving or receding from the 

 sphere 1, the latter will be slightly retarded ; and thus a suc- 

 cession of small decrements of velocity, forming the second 

 half of the wave, is transmitted in precisely the same manner 

 along the row of spheres : or the second half of the wave con- 

 sists in a peculiarity in the motion of the spheres such that 

 they move forward with a velocity somewhat less than the 

 normal velocity and backward with the normal velocity. 



6. It is of course clear, as before remarked, that in the case 

 of a gas the molecules in their mutual collisions would not all 

 be moving in the direct line of propagation of the wave at the 

 instant of its passage, but some of them more or less obliquely 

 to the line of propagation ; so that, for this cause, the rate of 

 propagation of the wave would be necessarily, to a certain 

 extent, slower than the normal velocity of the molecules them- 

 selves. This, however, does not affect in the least the prin- 

 ciple involved ; and therefore the above mode of illustration 

 serves to give a perfectly just idea of the physical process by 

 which, through the normal motion of the molecules of a gas, 

 changes of velocity experienced by the molecules or "waves" 

 are propagated to a distance through the gas in accordance 

 with the kinetic theory. 



7. Cause producing the Oscillation of the Mass of Air. — It is 



