in SPHERICAL BODIES. 365 
15. THIs is given merely as an example of the method of 
condudting the calculus when the variation of the denfity is 
taken into account, and not becaufe there is reafon to believe 
that the law which that variation actually follows, is the fame 
that has now been hypothetically affumed. 
16. THE principle on which we have proceeded, applies not 
only to folids, fuch as we fuppofe the interior of the earth, but 
it applies alfo to fluids like the atmofphere, provided they are 
fuppofed to have reached a fteady temperature. The propaga- 
tion of heat through fluids is indeed carried on by a law very 
different from that which takes place with refpedt to folids ; it 
is not by the motion of heat, but by the motion of the parts of 
the fluid itfelf. Yet, when we are feeking only the mean re- 
fult, we may fuppofe the heat to be fo diffufed, that it does not 
accumulate in any particular ftratum, but is limited by the 
equality of the momentary increments and decrements of tem- 
perature which that ftratum receives. This is conformable to 
experience ; for we know that a conftancy, not of temperature, 
but of difference between the temperature of each point in the 
‘atmofphere and on the furface, actually takes place. Thus, 
near the furface, an elevation of 280 feet produces, in this 
country, a diminution of one degree. The ftrata of our atmo- 
fphere, however, differ in their capacity of heat, or in the 
quantity of heat contained in a given fpace, at a given tempe- 
rature. Concerning the law which the change of capacity 
follows, we have no certain information to guide us ; and we 
have no refource, therefore, but to affume a hypothetical law, 
agreeing with fuch facts as are known, and, after deducing the 
refults of this law, to compare them with the obfervations made 
on the temperature of the air, at different heights above the 
furface of the earth. 
17. LEP 
