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 accurately known. The decrease of one degree for 172 meé- 
tres of difference in vertical elevation, as drawn from the aéros- 
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 temperature 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 pea of the fluid cannot surpass that of its li- 
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 surface ; the second is a necessary condition to the equi- 
librium of the Guid 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 suffice, 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- 
ereases 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 
