g58 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 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 
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, 
