442 PHYSIOLOGICAL PHYSICS. [Chap. xxxm. 



would give out a musical note, and if this were done 

 when the column of air was just the length to inten- 

 sify the sound of the fork it would be found that the 

 two sounds were precisely of the same pitch. The 

 column of air is of such a length as to vibrate in har- 

 mony with the fork. Consequently when the fork is 

 sounded in its neighbourhood it sets the air of the jar 

 in synchronous oscillation, and thus the intensity of 

 the sound is increased. The term applied to the 

 reinforcement of the sound is resonance, and the body 

 which reinforces it is called a resonator. If the 

 column of air be shortened or lengthened, its period 

 of vibration is no longer the same as that of the fork, 

 and the intensification is not produced. The column 

 of air in the jar will not vibrate in harmony with 

 any fork but that with which its vibrations agree. 

 If a fork of another pitch is brought near to its mouth 

 it is dumb. Now each limb of the fork moves to and 

 fro on each side of its position of rest. When the 

 limb moves outwards it produces a condensation of 

 the air in contact with it; when it moves inwards 

 a rarefaction is the result. The condensation travels 

 through the air in the jar till it reaches the surface of 

 the water, by which it is reflected. If the condensa- 

 tion reaches the mouth of the jar, on its return 

 journey, just when the outward excursion of the limb 

 of the fork is completed and its recoil is about to 

 begin, it is in time with it, and the succeeding rare- 

 faction will be propagated through the jar and will be 

 completed when the outward excursion of the fork 

 is about to be renewed, and a second condensation to 

 begin. The movements in the jar are thus in time 

 with the movements of the fork. The condensation 

 is half a wave length, and it travels through the 

 column of air in the jar twice, once downwards and 

 once upwards, so that the length of the column of 

 air in the jar is half that of a condensation, i.e. a 



