ILLUSTRATION OF RESONANCES AND ACTIONS OF A 

 SIMILAR NATURE 



[Journal of the Franklin Institute, XCIV, 275-278, 18721 



At the present day, when scientific education is beginning to take 

 its proper place in the public estimation, anything which can help 

 toward imparting a clear idea of any physical phenomenon becomes im- 

 portant. There are a number of these phenomena, of which resonance 

 is one, which play quite an important part in nature, but which as yet 

 have not been illustrated with sufficient clearness in the lecture-room. 

 Among these are the following: A person carrying water may so time 

 his steps as to produce waves which shall rise and fall in unison with 

 the motion of his body; soldiers in crossing a bridge must not keep 

 step, or they may transmit such a vibration to it as to break it down; 

 window-panes are sometimes cracked by sounding a powerful organ- 

 pipe to which they can vibrate ; a tuning-fork will respond to another of 

 equal pitch sounded near it; and others will readily suggest themselves 

 to the reader. In all these cases we have two bodies which can vibrate 

 in equal times, connected together either directly or by some medium 

 which transmits the motion from one to the other. We can, then, 

 readily reproduce the circumstances in the lecture-room. 



The vibrating bodies which I have found most convenient are pendu- 

 lums; they are easily made, are seen well at a distance, and their time 

 of vibration can be easily and quickly regulated. The apparatus can 

 be prepared in the following manner: Fix a board, about a foot long, 

 in a horizontal position; suspend a piece cf small stiff wire, of equal 

 length, beneath its edge, parallel to it, and an inch or two distant, by 

 means of threads. To one end of the board suspend a pendulum, con- 

 sisting of a thread about ten or twenty inches long, to which is attached 

 a ball weighing two or three ounces; join the thread of this pendulum 

 to the horizontal wire by taking a turn of it around the wire, so that 

 when the pendulum oscillates, it causes the wire to move back and 

 forth in unison with it. To complete the apparatus, prepare a number 

 of small pendulums by suspending bullets to threads, and let them have 

 small hooks of wire to hang by. 



