Scientific Lectures. 187 



vertically, you see the wave which runs along its whole length, and 

 then, meeting an obstacle in the hand of my assistant, who holds the 

 other end of it, is reflected, returns to me, is again reflected, and so 

 on, a number of times. But the particles of brass, in this experiment, 

 are moved neither forward nor backward, they simply rise and fall. 



Perhaps this fact may be made still more evident with a little appa- 

 ratus I devised years ago. "We now have on the screen a row of 

 small bright spots, intended to represent a line of material particles 

 at rest. I can communicate to them a wave-like motion ; you see 

 distinct waves advancing along their whole length, and, although 

 these waves are so plain, pray notice that each particle merely rises 

 and falls, in suitable time. Hence, to sum up, in the case of a wave 

 of water, it is the motion which advances, not the material, and this 

 advance of the motion may be brought about, as we have seen, by a 

 movement of the material particles at right angles to the direction 

 of the advancing wave. 



Strangely enough, there is another kind of motion, which can be 

 communicated to a row of material particles, which also will generate 

 along them waves, but waves of a different character. In these new 

 waves, which will give us so much occupation to-night, the particles 

 are not pushed aside out of line, as in waves of water or light, but at 

 one instant are crowded together, and afterward fly apart, remaining, 

 however, always in the same original line. I have contrived a little 

 piece of apparatus which will make the nature of this singular action 

 evident. We have upon the screen a row of spots of light, represent- 

 ing material particles, and you readily see how this alternate condensa- 

 tion and expansion affects them, and it is plain that they are 

 transmitting a series of wave-like impulses. If you select any indi- 

 vidual particle for observation, you easily notice that, at one instant, 

 it presses nearer its neighbors, and the next retreats from them, each 

 particle beginning and ending its swaying motion in such time that 

 the general effect is that of a wave in motion. I gave an experi- 

 mental illustration of the first class of waves with the brass cord, and 

 will now add that its spirals will just as easily transmit the waves we 

 are now considering, though, of course, you do not see them. If you 

 should hold the end of the cord, you would, however, be able to feel 

 them, for, when they struck your hand, they would give it a little 

 push and a pull. This thin elastic box will answer our purpose. I 

 attach the end of the cord to it, and send along it one of these waves. 

 As it strikes the box, a sharp, distinct sound is produced, in fact a 

 series, owing to repeated reflections of the wave, which I generate 



