230 On Warming and Ventilating 



infinite variety of currents. If the substance of the earth were 

 a much better conductor of heat, we should experience less 

 extremes of heat and cold upon its surface. The summer-heat 

 would be more rapidly absorbed by the earth, and the rigour of 

 winter would be much diminished by the heat derived from the 

 earth in the sun's absence. The nature of soils, as regards their 

 conducting power, has doubtless a great influence in limiting 

 the extremes of temperature in winter and summer. The heat 

 produced on any part of the earth's surface, will be the greatest 

 where the rays of the sun are vertical, and the surface of 

 such a nature as to receive the rays with the greatest fa- 

 cility, its substratum being, at the same time, the worst con- 

 ductor of heat. The air immediately in contact with this sur- 

 face becomes heated, and specifically lighter than its super- 

 stratum. This causes, in the first instance, two simultaneous 

 currents; one perpendicularly upwards, and the other, a lateral 

 one from all the surrounding parts towards the centre of the 

 heated surface. After the ascending current has attained a 

 certain altitude, it progressively assumes an oblique and ulti- 

 mately a lateral direction, but in an inverse order to that of 

 the lower stratum. By this beautiful, provision of natural eco- 

 nomy, the heated air of the torrid zone, and the chilling cur- 

 rents from the polar regions mutually contribute to the preven- 

 tion of those extremes of heat and cold, which would otherwise 

 be fatal to every class of animated beings. 



To form some idea of the effect which would result from 

 a vertical sun upon a good reflecting surface, such as a black 

 soil, unattended by the currents of air above alluded to, we 

 have only to observe the heat generated in hot-houses ; in which 

 case the heated air is to a certain degree prevented from as- 

 cending, and consequently the lateral current from coming in. 

 The heat produced by these means, therefore, Avill be greater 

 in proportion to the blackness and lightness of the soil, to the 

 tightness of the surrounding walls and windows, and the per- 

 pendicularity of the sun's rays. Hence we see the importance 

 ofour atmosphere independently of its agency in respiration. 

 Without it, bodies would receive their heat on those parts only 



