415 
Mr. Ivory on the astronomical refractions. 
degrees of cold, the law of dilatation and contraction may 
be very different from what it has been observed to be in the 
limited range of our experiments. This observation is pro- 
bably well founded, but it will not destroy the force of what 
has been advanced. We know that air always gives out heat 
when it is compressed into a less volume, and absorbs heat 
when it expands. As long therefore as that fluid retains its 
elasticity, so long, we must conclude, will temperature con- 
tinue to modify the changes of bulk which that force produces. 
The law of dilatation and contraction may no doubt undergo 
some change in different circumstances, but every expansion 
must be productive of cold, and every new degree of cold 
must diminish the elastic force of a given volume of air. 
Gravity continuing to act with nearly the same energy, while 
the elastic force of the air is continually diminished, these 
two forces will at length become equivalent, and will counter- 
balance one another, which is all that is necessary for imposing 
a limit to the extent of the atmosphere. We have proved that 
air, if it were confined by the action of gravity alone, would 
extend indefinitely into space ; and it is not unreasonable to 
consider the effect of temperature as a contrivance for securely 
attaching to the terrestrial globe a fluid so necessary in every 
point of view to the economy of nature. 
Since it is found that all elastic fluids follow the same laws 
in regard to heat and pressure, the foregoing reasoning is 
equally true, whether we conceive the atmosphere as com- 
posed of one homogeneous fluid, or as a collection of many 
elastic gases and vapours, however much they may differ 
from one another in specific gravity. 
It may even be possible to form some reasonable conjecture 
