390 FKAGMENTS OF SCIENCE. 



moves it a little way from its position of rest ; it swings 

 back towards me, and when it reaches the limit of its 

 swing I puff again. It now swings further ; and thus by 

 timing the puffs I can so accumulate their action as to 

 produce oscillations of large amplitude. The ivory ball 

 here has absorbed the motion which my breath com- 

 municated to the air. I now bring the ball to rest. 

 Suppose, instead of the breath, a wave of air to strike 

 against it, and that this wave is followed by a series of 

 others which succeed each other exactly in the same 

 intervals as my puffs ; it is obvious that these waves 

 would communicate their motion to the ball and cause 

 it to swing as the puffs did. And it is equally manifest 

 that this would not be the case if the impulses of the 

 waves were not properly timed ; for then the motion 

 imparted to the pendulum by one wave would be neutra- 

 lised by another, and there could not be the accumula- 

 tion of effect obtained when the periods of the waves 

 correspond with the periods of the pendulum. So much 

 for the particular impulses absorbed by the pendulum. 

 But if such a pendulum set oscillating in air could pro- 

 duce waves in the air, it is evident that the waves it 

 would produce would be of the same period as those 

 whose motions it would take up or absorb most com- 

 pletely, if they struck against it. 



Perhaps the most curious effect of these timed 

 impulses ever described was that observed by a watch- 

 maker, named Ellicott, in the year 1741. He left two 

 clocks leaning against the same rail; one of them, 

 which we may call A, was set going ; the other, B, not. 

 Some time afterwards he found, to his surprise, that B 

 was ticking also. The pendulums being of the same 

 length, the shocks imparted by the ticking of A to the 

 rail against which both clocks rested were propagated 

 to B, and were so timed as to set B going. Other 



