ILLUSTRATION OF KESONANCES 29 



Having then set the heavy pendulum in motion, hang some of the 

 light ones on the horizontal wire, and note the result: those which are 

 shorter or longer than the heavy one will not be affected, but if any of 

 them are nearly of the same length, they will begin to vibrate to a 

 small extent, but will soon come to rest, after which they will com- 

 mence again, but stop as before ; but if any one happens to be of exactly 

 the proper length, its motion will soon become very great, and im- 

 mensely surpass in amplitude that of the heavy one, although the motion 

 is derived from it. Of course the heavy pendulum must be retarded in 

 giving motion to the light one, but it is hardly perceptible when there is 

 great difference in the weight. In the same manner a tuning-fork will 

 undoubtedly come to rest sooner when producing resonance than when 

 vibrating freely. To show this retardation more clearly, suspend two 

 pendulums, equal in weight and length, to the edge of a horizontal 

 board, and connect their two threads together by a horizontal thread 

 tied to each at a point an inch or two from the top, and drawn so tight 

 as to pull each of the pendulums a little out of plumb. On starting one 

 of these pendulums the other will gradually move, and finally absorb 

 all the motion from the first, and bring it entirely. to rest; the action 

 will then begin anew, and the motion will be entirely given back to the 

 first ball. This experiment differs from that of resonance, inasmuch 

 as in the case of the pendulums all the motion of the first ball is finally 

 stored up in the second; but in the case of resonance the confined air 

 is constantly giving out its motion to the atmosphere in waves of sound. 

 To imitate this to some extent we must attach a rather large piece of 

 paper to the second pendulum, so that it will meet with resistance, and 

 then both balls will come to rest sooner than otherwise. If one of the 

 balls is only two or three times heavier than the other, they will then 

 also interchange motions; but when the heavy ball has the motion, 

 the arc of its vibration will not be so great as that of the other when 

 it vibrates. 



To illustrate the use of Helmholtz resonance globes, or Koenig's 

 apparatus for the analysis of sounds, we can enlarge and modify the 

 first apparatus somewhat. Make the board six or eight feet long, and 

 suspend at one end four or five of the heavy pendulums, and at the 

 other the same number of light ones, each of which corresponds in time 

 of vibration with one of the heavy ones. On now causing any of the 

 heavy pendulums to vibrate, as No. 3, we shall meet with no response 

 from any of the light ones except No. 7. If Nos. 1, 2 and 4 are set 

 going at one time, the wire A will be drawn hither and thither by the 



