5i; 



Article XVI. 

 On Barometrical Measurement of Heights. By F. W. Bessel. 



[From the Adrono7n'ische Nachrichten, Nos. 356, 357.] 

 1. 



1 HE atmosphere of the earth is known to be composed of 

 the nitrogen, oxygen, and carbonic acid gases, and of aqueous 

 vapour. These constituents are supposed to exercise no che- 

 mical action on each other; and arbitrary quantities of them, 

 mixed together under circumstances of equal temperature and 

 pressure, occupy spaces equivalent to the sum of the spaces that 

 they would severally occupy. Were we to assume that the 

 constituents of the atmosphere are mixed in the same propor- 

 tion at all times and at all altitudes, we might dispense with the 

 knowledge of what that proportion is, in treating of the condi- 

 tions of their equilibrium ; but if we desire to preserve the 

 freedom of founding our investigations on other suppositions 

 also, we must not pass by in silence the mode in which the con- 

 stituents are combined. 



The proportion of the three gases may not always be exactly 

 the same at a given point of the earth's surface ; but the altera- 

 tions which take place are so small, that they are only discover- 

 able by chemical experiments frequently repeated ; we cannot, 

 therefore, regard the proportion as determinable by observation for 

 each particular case, and we must assume a certain proportion. 



According to Berzelius, the spaces occupied by the three 

 gases, in the order in which they are named above, are to each 

 other as 



77-96; 21-15; 0-07; 



or, one volume of dry atmospheric air at the surface of the 



earth contains 



V = 0*78605 nitrogen gas 



V, =0*21325 oxygen gas 



v^ = 0*00070 carbonic acid gas. 



The same great chemist has given the densities of these three 

 gases, under the pressure which gives to the mixture the den- 

 sity D, viz. : 



VOL. II. PART VIII. 2 M 



