338 FKAGMENTS OF SCIENCE. 



we seize this idea definitely, we shall have no difficulty 

 in dropping the metaphor of springs, and substituting 

 for it mentally the forces by which the atoms act upon 

 each other. Having thus far cleared our way, let us 

 make another effort to advance. 



A heavy ivory ball is here suspended from a string, 

 I blow against this ball; a single puff of my breath 

 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 

 communicated 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 neu- 

 tralized by another, and there could not be the accumu- 

 lation 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 im- 

 pulses ever described was that observed by a watch- 

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



