25 



The flame of camphire is white ; of sulphur, blue; of 

 white-wax, inclining to yellow. For making experi- 

 ments, oil may be impregnated with different metals, so 

 as to exhibit their particular flames. 



4. Air is the ordinary vehicle of sound, which is the 

 fainter, the more remote the sounding body is. It is 

 also lessened, and sometimes qui f e interrupted either by 

 contrary winds or thick vapours floating in the air. It is 

 supposed, that the sounding body, excites a kind of un- 

 dulation or tremulous motion in the air, raising as it 

 were waves of air, one of which impels the other till 

 they reach the ear. 



Sound moves but little quicker by having the wind 

 vMi it, as it moves at least thirty three times faster titan 

 the most violent wind we know. But it is heard much 

 farther thereby. 



That air is the grand vehicle of soud, appears from 

 various experiments. A. bell in an unexhausted receiver, 

 may be heard at some distance; but scarce at the 

 smallest, when it is exhausted. But it is not the only 

 one, water too will convey sound. If you strike a bell 

 under water, the sound is heard plain, only not so loud, 

 and also a fourth deeper. And a sound made in air, is 

 heard under water, with just the same difference. 



Sounds commonly move a mile in about nine se- 

 conds and a quarter. If a gun be discharged with its 

 mouth to us or from us, the report comes to us in the 

 very same time. It always moves the nearest way, and 

 equally swift from the beginning to 4lie end of its mo- 

 tion. 



If the undulating air strikes against hard concave 

 bodies, it rebounds, and occasions what we call an echo. 

 As often as sound strikes perpendicularly on a wall, be- 

 hind which is any vault or arch, or even a parallel wall, 

 so often it will be reverberated in nearly the same line. 

 For a multiplied echo, there must be a number of walls 

 and cavities, either behind, or fronting each. other. 



The echo in Woodstock-park returns < T ery distinctly, in 



