and of the Planetary Spaces. 67 



In relation to the physical constitution of the atmosphere, the 

 laws of the decrease of the quantity of vapour, of density, and 

 of temperature, proportioned to elevation above the horizon are 

 not accm-ately known. The decrease of one degree for 172 me- 

 tres of difference in vertical elevation, as drawn from the aeros- 

 tatick experiments of M. Gay Lussac refers to the temperature 

 indicated by the thermometer suspended in the open air, but this 

 does not disclose to us that of the layers of air themselves, of 

 which the actual temperature determines the radiation, and per- 

 haps exercises an influence upon the absorbent power. All we 

 know, in this repect, is, that the mean temperatme of the air in 

 contact with the surface of the globe should equal that of the 

 surface itself; and that, at the superiour limit of the atmosphere, 

 the actual temperature of the fluid cannot surpass that of its li- 

 quefaction to such degree as that the density should be reduced. 

 The first condition results, as previously stated, from the contin- 

 ual contact of the inferiour layer of the atmosphere with the 

 earth's siu:face ; the second is a necessary condition to the equi- 

 librium of the fluid mass, and independent of the general equa- 

 tion of this equilibrium. 



In fact, if we divide this mass into concentrick layers so infi- 

 nitely thin that the weight of each layer should be insensible, the 

 weight of an interiour layer would suflice, nevertheless, to coun- 

 terpoise the difference of pressure which would be exercised, in 

 opposite directions, upon these two faces, which has for its extent 

 the elastick forces of the two adjacent layers ; but, the most ele- 

 vated layer experiencing no pressure upon its superiour face, could 

 not, by its weight, balance the pressure exercised upon the other 

 face, if this were appreciable ; consequently the elastick force of 

 the air should be nothing at the limit of the atmosphere, of which 

 the distance, from the surface of the earth, is much less than the 

 distance at which its centrifugal force would destroy its gravity. 

 Now, the elastick force cannot be reduced to zero, because it de- 

 creases only in the ratio of the density, according to the law of 

 Mariotte ; and however reduced, therefore, the density of the air 

 may be, it will retain an elastick force by virtue of which it will 

 be still farther dilated ; and the atmosphere thus deprived of any 

 fixed limit, would extend through all space. It cannot be ob- 

 jected that the atmosphere would be maintained in place by the 

 pressure of the ether upon its superiour surface ; for this ether 



