308 POPULAR LECTURES AND ADDRESSES. 



is a spreading out of condensation from the place 

 where my hand moves in one direction and then in 

 the reverse. Each condensation is succeeded by 

 a rarefaction. Rarefaction succeeds condensation 

 at an interval of one-half what we call " wave- 

 lengths." Condensation succeeds condensation at' 

 the full interval of a wave-length. 



We have here these luminous particles on this 

 scale, 1 representing portions of air close together, 

 more dense ; a little higher up, portions of air 

 less dense. I now slowly turn the handle of 

 the apparatus in the lantern, and you see the 

 luminous sectors showing condensation travelling 

 slowly upwards on the screen ; now you have 

 another condensation making one wave-length. 



This picture or chart represents a wave-length 

 of four feet. It represents a wave of sound four 

 feet long. The fourth part of a thousand is 250. 

 What we see now of the scale represents the lower 

 note C of the tenor voice. The air from the mouth 

 of a singer is alternately condensed and rarefied 



1 Alluding to a moving diagram of wave motion of sound pro- 

 duced by a working slide for lantern projection. 



