in SPHERICAL BODIES. 3 6 5 



15. This is given merely as an example of the method of 

 conducting 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 alTumed. 



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 fleady temperature. The propaga- 

 tion of heat through fluids is indeed carried on by a law very 

 different from that which takes place with refpect to folids j 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- 

 mit, 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 arid decrements of tem- 

 perature which that ftratum receives. This is conformable to 

 experience > for we know that a conflancy, 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 afliime a hypothetical law, 

 agreeing with fuch fads as are known, and, after deducing the 

 reiults 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, Let 



