PHYSICAL BASIS OF SOLAR CHEMISTRY 357 



then the motion imparted to the pendulum by one wave 

 would be neutralized 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 pen- 

 dulum. But if such a pendulum set oscillating in air could 

 produce 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 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 ob- 

 served. 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 



