AT THE EARTH'S SURFACEi 413 



tingent, requiring the presence of another condition, which 

 may be wanting, and actually is wanting, in many instances ; — 

 this is, that the quantity of heat in the system should be given, 

 and should not admit of continual increase from one quarter, 

 nor diminution from another. When such increase and dimi- 

 nution take place, no such equilibrium can be attained. In 

 proof of this, he mentions the fact, that a bar of iron thrust 

 into the fire, though red-hot at one extremity, will not become 

 so at the other in any length of time, but each part of it will 

 have a fixed temperature, lower as it is farther from the fire, 

 but remaining invariable while the condition of the fire, and 

 of the surrounding medium, continues the same. He illustrates 

 it also more fully by the following example : Let A, B, C, D, 

 &c. be a series of contiguous bodies, or let them be parts of 

 the same body ; and let us suppose that A receives from some 

 cause, into the nature of which we are not here to inquire, a 

 constant and uniform supply of heat. It is plain that heat will 

 flow continually from A to B, from B to C, &c. ; and in order 

 that this may take place, A must be hotter than B, B than C, 

 and so on ; so that no uniform distribution of heat can ever 

 take place. The state to which the system will tend, and at 

 which, after a certain time, it must arrive, is one in which the 

 momentary increase of the heat of each body is just equal to 

 its momentary decrease, so that the temperature of each indi- 

 vidual body becomes fixed, all these temperatures together 

 forming a series decreasing from A downwards. This is then 

 applied to the argument in the following manner : " If heat be 

 communicated to a solid mass, like the Earth, from some 

 source or reservoir in its interior, it must go off from the 

 centre on all sides towards the circumference. On arriving at 

 the circumference, if it were hindered from proceeding farther, 

 and if space or vacuity presented to heat an impenetrable bar- 



Vol, VII. P. II. 3 G 



rier, 



