120 TERRESTRIAL ADAPTATIONS. 



fluid in which they take place, but also, many 

 may be propagated in the same fluid at the same 

 time, without disturbing each other. We may 

 see this effect on water. When several drops 

 fall near each other, the circles which they pro- 

 duce cross each other, without either of them 

 being lost, and the separate courses of the rings 

 may still be traced. 



All these consequences, both in water, in air, 

 and in any other fluid, can be very exactly 

 investigated upon mechanical principles, and the 

 greater part of the phenomena can thus be shown 

 to result from the properties of the fluids. 



There are several remarkable circumstances 

 in the way in which air answers its purpose as 

 the vehicle of sound, of which we will now point 

 out a few. 



2. The loudness of sound is such as is con- 

 venient for common purposes. The organs of 

 speech can, in the present constitution of the air, 

 produce, without fatigue, such a tone of voice as 

 can be heard with distinctness and with comfort. 

 That any great alteration in this element might 

 be incommodious, we may judge from the diffi- 

 culties to which persons are subject who are dull 

 of hearing, and from the disagreeable effects of a 

 voice much louder than usual, or so low as to be 

 indistinct. Sounds produced by the human 

 organs, with other kinds of air, are very different 

 from those in our common air. If a man inhale 



