Prof. Tyndall on the Physical Basis of Solar Chemistry. 153 



imparted to the pendulum by one wave would be neutralized by an- 

 other, and there could not be that accumulation of effect which we 

 have when the periods of the waves correspond with the periods of 

 the pendulum. So much for the kind of impulses absorbed by the 

 pendulum. But such a pendulum set oscillating in air produces 

 waves in the air ; and we see that the waves which it produces must 

 be of the same period as those whose motions it would take up or 

 absorb most copiously if they struck against it. Just in passing I 

 may remark that, if the periods of the waves be double, treble, 

 quadruple, &c. the periods of the pendulum, the shocks imparted to 

 the latter would also be so timed as to produce an accumulation of 

 motion. 



Perhaps the most curious effect of these timed impulses ever de- 

 scribed was that observed by a watchmaker named Ellicott, in the 

 year 1741. He set 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 curious 

 effects were at the same time observed. When the pendulums dif- 

 fered from each other a certain amount, A set B going, but the reac- 

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

 stopped B. If 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 mutual compromise they ticked in perfect unison. 



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

 mechanically identical. 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 eethereal waves stri- 

 king upon atoms which oscillate in the same periods as the waves 

 succeed each other, the motion of the waves will be absorbed by the 

 atoms ; suppose we send our beam of white light through a sodium 

 flame, the particles of that flame will be chiefly affected by those un- 

 dulations 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 sether to the atoms of the volatilized sodium, 

 which, as already defined, is absorption. We use glass screens to 

 defend us from the heat of our fires; how do they act? Thus : — 

 The heat emanating from the fire is for the most part due to radia- 

 tions which are incompetent to excite the sense of vision ; we call 

 these rays obscure. Glass, though pervious to the luminous rays, 

 is opake in a high degree to those obscure rays, and cuts them off, 

 while the cheerful light of the fire is allowed to pass. Now mark 

 me clearly. The heat cut off from your person is to be found in the 

 glass, the latter becomes heated and radiates towards your person ; 

 what, then, is the use of the glass if it merely thus acts as a tempo- 

 rary halting-place for the rays, and sends them on afterwards? It 

 does this : — it not only sends the heat it receives towards you, but 



