in SPHERICAL BODTES. 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‘. It is of this general tendency, 
that the equilibrium or uniform diftribution 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 inttances. This condition is no other, than 
that the quantity of heat in the fyftem fhould be given, and 
fhould 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 exifts. Thus, if we expofe a thermometer to the 
fun’s rays, it immediately rifes, and continues to ftand 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 inftant from the folar rays. When this happens, its tempe- 
ature becomes ftationary ; the momentary increment and de- 
crement of the heat are the fame, and the total, of courfe, con- 
tinues conftant. 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- 
fifts between the increments and the decrements juft mention- 
ed: thefe indeed are, ftrictly fpeaking, zm 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. 
J Yy 2 3. In 
