WAVE THEORY OF LIGHT 265 



ten times, that is, twenty times per second, which would 

 require four times as much force; three times ten, or 

 thirty times a second, would require nine times as much 

 force, If a person were nine times as strong as the most 

 muscular arm can be, he could vibrate his hand to and 

 fro thirty times per second, and without any other musi- 

 cal instrument could make a musical note by the move- 

 ment of his hand which would correspond to one of the 

 pedal notes of an organ. 



If you want to know the length of a pedal pipe, you can 

 calculate it in this way. There are some numbers you must 

 remember, and one of them is this. You, in this coun- 

 try, are subjected to the British insularity in weights and 

 measures ; you use the foot and inch and yard. I am obliged 

 to use that system, but I apologise to you for doing so, 

 because it is so inconvenient, and I hope all Americans 

 will do everything in their power to introduce the French 

 metrical system. I hope the evil action performed by 

 an English minister whose name I need not mention, be- 

 cause I do not wish to throw obloquy on any one, may 

 be remedied. He abrogated a useful rule, which for a 

 short time was followed, and which I hope will soon be 

 again enjoined, that the French metrical system be taught 

 in all our national schools. I do not know how it is in Amer- 

 ica. The school system seems to be very admirable, and 

 I hope the teaching of the metrical system will not be let 

 slip in the American schools any more than the use of 

 the globes. I say this seriously: I do not think any one 

 knows how seriously I speak of it. I look upon our English 

 system as a wickedly brain-destroying piece of bondage 

 under which we suffer. The reason why we continue to 

 use it is the imaginary difficulty of making a change, and 

 nothing else; but I do not think in America that any such 

 difficulty should stand in the way of adopting so splendidly 

 useful a reform. 



I know the velocity of sound in feet per second. If 

 I remember rightly, it is 1089 feet per second in dry air 

 at the freezing temperature, and 1115 feet per second in 

 air of what we would call moderate temperature, 59 or 60 de- 

 grees (I do not know whether that temperature is ever 



