BELL IN EXHAUSTED RECEIVER. 113 



MRS. F. 



Send an umbrella to Mary, and when she arrives I will 

 explain it to you. 



In a few minutes Mary returned, and, the party being 

 seated, Mrs. Fortescue began her promised conversation, by 

 asking who could tell what is the vehicle by which sounds 

 are commonly conveyed. 



HENRIETTA. 



The air, I suppose. 



MRS. F. 



It is by the air, certainly, that sounds are conveyed; but 

 this important fact was not established until the beginning of 

 the last century. 



ESTHER. 



And how was it then ascertained 1 ? 



MRS. F. 



By the simple experiment of suspending a bell in a glass 

 vessel; as the air was gradually drawn out, the sound became 

 fainter and fainter, until the vessel was completely exhausted, 

 when the sound could no longer be heard. Upon re-admit- 

 ting the air, the sound was, of course, again heard; and on 

 forcing more air into the vessel than is equal to the atmos- 

 pheric pressure, the loudness of the sound increased in like 

 proportion. 



ESTHER. 



Then now I understand why such a deep silence appears 

 to reign in the elevated parts of the globe. All travellers 

 who have ascended to any considerable height, speak of the 

 universal stillness which pervades. 



FREDERICK. 



How does my aunt's experiment account for that? 



ESTHER. 



In this manner; because the higher we ascend, the rarer 

 10* 



