TEMPEllATUKE OF THE SUKFACE. 411 



glowing heat of tlie inferior strata and tlie universal chaitep. 

 regions of space, whose temperature is prohablj' below -^^^''l'- 

 the freezing point of mercury. 



'' The periodic changes of temperature which have renctrutlon 

 been occasioned on the earth's surface by the sun's posi- t'mnperature. 

 tion and by meteorological processes, are continued in its 

 interior, although to a very inconsiderable depth. The 

 slow conducting power of the ground diminishes this loss 

 of heat in the winter, and is very favourable to deep- 

 rooted trees. Points that lie at very different depths on 

 the same vertical line attain the maximum and mini- 

 mum of the imparted temperature at very different 

 periods of time. The further they are removed from the 

 surface the smaller is this difference between the ex- 

 tremes. In the latitudes of our temperate zone (between 

 48° and 52°) the stratum of invariable temperature is stratum of 

 at a depth of from 59 to 64 feet, and at half that depth [e\npL,','.'iJure 

 the oscillations of the thermometer, from the influence 

 of the seasons, scarcely amount to half a degree. In tro- 

 pical climates this invariable stratum is only one foot 

 below the surface, and this fact has been ingeniously 

 made use of by Boussingault to obtain a convenient, and, 

 as he believes, certain determination of the mean tem- 

 perature of the air of different places. This mean tem- 

 perature of the air at a fixed point, or at a group of con- 

 tiguous points on the surface, is to a certain degree the 

 fundamental element of the climate and agricultural re- 

 lations of a district ; but the mean temperature of the 

 whole surflice is very different from that of the globe 



itself. The questions so often agitated, whether the Changes of 



, , . , • 1 1 1 J- J- tcmiierature. 



mean temperature has experienced any considerable dii- 



ferences in the course of centuries, whether the climate 

 of a country has deteriorated, and whether the winters 

 have not become milder and the summers cooler — can 

 only be answered by means of the thermometer ; this 

 instrument has, however, scarcely been invented more 

 than two centuries and a half, and its scientific applica- 

 tion hardly dates back 120 years. The nature and no- 



