258 FRAGMENTS OF SCIENCE. 



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 neutral- 

 ized by another, and there could not be the accumulation 

 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 produce waves 

 in the air, it is evident that the waves it would product; 

 would be of the same period as those whose motions it 

 would take up or absorb most completely, if they struck 

 against it. 



Perhaps the most curious effect of these timed impulses 

 ever described was that observed by a watchmaker, 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 afterward 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 curious effects were at the same time 

 observed. When the pendulums differed from each other 

 a certain amount, A set B going, but the reaction of B 

 stopped A. Then B set A going, and the reaction of A 

 stopped B. When the periods of oscillation were close to 

 each other, but still not quite alike, the clocks mutually 

 controlled each other, and by a kind of compromise they 

 ticked in perfect unison. 



But what has all this to do with our present subject? 

 The varied actions of the universe are all modes of motion; 

 and the vibration of a ray claims strict brotherhood with 

 the vibrations of our pendulum. Suppose ethereal waves 

 striking upon atoms which oscillate in the same periods as 

 the waves, the motion of the waves will be absorbed by the 

 atoms; suppose we send our beam of white light through 

 a sodium flame, the atoms of that flame will be chiefly 

 affected by those undulations which are synchronous with 

 their own periods of vibration. There will be on the part 

 of those particular rays a transference of motion from the 

 agitated ether to the atoms of the volatilized metal, which, 

 as already defined, is absorption, 



