Chap. 9] i>f the Atmofyhere* 43$ 



' It was not however at any time fuppofed, that this 

 calculation could be juft; for as the air is an elaf- 

 tic fluid, the upper parts muft expand to an immenfe 

 bulk, and thus render the calculation above related ex- 

 ceedingly erroneous. By experiments made in diffe- 

 rent countries, it has been found that the fpaces, which 

 any portion of air takes up, are reciprocally propor- 

 tional to the weight with which it is compreffed. Al- 

 lowances were therefore to be made in calculating the 

 height of the atmofphere. If we fuppofe the heighc 

 of the whole divided into innumerable equal parts, the 

 denfity of each of which is as its quantity, and the 

 weight of the whole incumbent atmofphere being alfo 

 as its quantity, it is evident, that the weight of the in- 

 cumbent air is every where as the quantity contained 

 in the fubjacent part, which makes a difference be- 

 tween the weights of each tv/o contiguous parts of air. 

 By a theorem in geometry, where the differences of 

 magnitudes are geometrically proportional to the mag- 

 nitudes themlelves, it appears that thefe magnitudes 

 are in continual arithmetical proportion j therefore, i 

 according to the ftippofition, the altitudes of the air, 

 by the addition of new parts into which it is divided, 

 do continually increafe in arithmetical proportion, its 

 denfity will be diminimed, or (which is the fame 

 thing) its gravity decreafed in continual geometrical 

 proportion. 



* It is now eafy, from ilrch a feries, by making two 



or three barometrical obfervations, and determining 



the denfity of the atmofphere at two or three different 



frations, to determine its abfolute height, or its rarity 



at any afiignable height. Calculations accordingly 



'were made upon this plan j but it having been found 



that the barometrical obfervaticns by no means cor- 



relponded with the denfity which, by other experi- 



F f 2 ments, 



