THE WAVE THEORY OF LIGHT, 305 



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 degrees (I do 

 not know whether that temperature is ever at- 

 tained in Philadelphia or not ; I have had no 

 experience of it, but people tell me it is some- 

 times 59 or 60 degrees in Philadelphia, and I 

 believe them) in round numbers let us call the 



x 



