in SPHERICAL BODIES. 355 



is higher, to thofe where it is lower, or from thofe which con 

 tain more heat, according to the indication of the thermome- 

 ter, to thofe which contain le r s. It is of this genera! tendency, 

 that the equilibrium or uniform diflribution of heat is a confe- 

 quence, — but a confequence only contingent, requiring the pre- 

 fence of another condition, which may be wanting, and actually 

 is wanting, in many instances. This condition is no other, than 

 that the quantity of heat in the fyfcem mould be given, and 

 mould not admit of continual increafe from one quarter, nor 

 diminution from another. When fuch increafe and diminution 

 take place, what is ufually called " the equilibrium of heat" 

 no longer exifls. Thus, if we expofe a thermometer to the 

 fun's rays, it immediately rifes, and continues to Hand above 

 the temperature of the furrounding air. The way in which 

 this happens is perfectly underftood : the mercury in the 

 thermometer receives more heat from the folar rays than 

 the air does | it begins therefore to rife as foon as thofe 

 rays fall on it ; at the fame time, it gives out a portion of 

 its heat to the air, and always the more, the higher it rifes. 

 It continues to rife, therefore, till the heat which it gives out 

 every inftant to the air, be equal to that which it receives eve- 

 ry inflant from the folar rays. When this happens, its tempe- 

 rature becomes ftationary ; the momentary increment and de- 

 crement of the heat are the fame, and the total, of courfe, con- 

 tinues conflant. The thermometer, therefore, in fuch circum- 

 ftances, never acquires the temperature of the furrounding air > 

 and the only equilibrium of the heat, is that which fub- 

 fifls between the increments and the decrements juft mention- 

 ed : thefe indeed are, ftrictly fpeaking, in equilibrio, as they ac- 

 curately balance one another. This fpecies of equilibrium, 

 however, is quite different from what is implied in the uni- 

 form diffufion of heat. 



Yy 2 3. In 



