NEWTON S PRINCIPIA. 281 



But we have no space to do more than merely allude to 

 their labours. 



There are a great many other waves propagated through 

 the air besides those which can be heard. Our ear is a 

 musical instrument that vibrates in consonance with all 

 notes whose periods lie between two limits. All other 

 sound waves therefore, though their theory is exactly the 

 same, are not heard. 



Besides these sound waves, there are other great waves 

 which traverse the air in all directions, and whose passage 

 is indicated by the variations of the barometer. These 

 waves are very large, and pass over whole continents in 

 their course, and seem to recur with some regularity. 

 Their theory is different from that of sound, and as yet 

 imperfectly understood. The examination of the &quot; facts &quot; 

 connected with them was begun by Herschel, and has been 

 since continued with much success by William Birt. 



So also the waves that produce sound can travel through 

 other substances besides air. The theory is in some points 

 different from that we have been considering. 



3. That the elevations on the surface of water which we 

 call waves are not really the transmission of a body of 

 water, but merely the movement of a particular state of 

 motion, will be obvious on very little consideration. 

 First) by experiment, a piece of cork floating on water 

 agitated in this manner is found merely to oscillate, and 

 does not advance as the wave moves onwards. Secondly, 

 by theory, such a disturbance being supposed given to the 

 water, as we know by experience will produce the appear 

 ance of waves, the principles of Hydrodynamics lead us 

 to the conclusion that the motion is really of a vibratory 

 kind. 



