AT THE EARTH'S SURFACE. 427 



at the latter, its effects would be more considerable in retard- 

 ing a farther augmentation of temperature. Thus it appears, 

 that the causes counteracting rise of temperature at the sur- 

 face of the globe, act most powerfully at those parts where it 

 is high, and any progressive rise, therefore, must be principal- 

 ly at those where it is low. 



The effect of this arrangement may be most obvious, per- 

 haps, from comparing the two extremes. At the equator, the 

 ascending current of warm air, carrying off heat, is constant 

 or nearly so ; towards the poles, the descending current im- 

 parting heat, must be equally constant. The heat directly 

 communicated at the former by the solar rays, and the heat 

 communicated at the latter by the current of warm air, will 

 both be in part conveyed through the solid mass towards the 

 interior ; but this will always be with a decreasing temperature 

 as it proceeds, — that is to say, the first or exterior layer will be 

 at a higher temperature than the second, the second than the 

 third, and the accumulation, therefore, will be at the surface, 

 to a certain extent. At the poles this may continue to pro- 

 ceed, because heat is there communicated without an equal ab- 

 straction. But at the equator it will remain stationary, or 

 nearly so, as no increase can take place without the abstrac- 

 tion both by the atmosphere and by radiation becoming pro- 

 portionally greater. 



This is aided by another effect, that with regard to the dif- 

 fusion of heat through the solid mass itself. This diffusion 

 from the surface proceeds in part towards the centre, or each 

 layer, as the surface is receded from, receives a portion of heat 

 from the exterior one, and this continues onwards, though with 

 a gradual diminution. But there must be at the same time a 

 diffusion more nearly horizontal, proceeding through these 



layers 



