MEAN TEMPERATURE OP THE EARTH. 175 



This loss experienced by the central heat must have been verj 

 considerable in the earliest epochs of the Earth's revolutions, 

 but within historical periods it has hardly been appreciable 

 by our instruments. The surface of the Earth is therefore 

 situated between the glowing heat of the inferior strata and 

 the universal regions of space, whose temperature is probably 

 below the freezing-point of mercury. 



The periodic changes of temperature which have been 

 occasioned on the Earth's surface by the Sun's position and 

 by meteorological processes, are continued in its interior, 

 although to a very inconsiderable depth. The slow conduct- 

 ing power of the ground diminishes this loss of heat in the 

 winter, and is very favorable to deep-rooted trees. Points 

 that lie at very different depths on the same vertical line 

 attain the maximum and minimum of the imparted tempera- 

 ture at very different periods of time. The further they are 

 removed from the surface, the smaller is this difference be- 

 tween the extremes. In the latitudes of our temperate zone 

 (between 48^ and 52°), the stratum of invariable temperature 

 is at a depth of from 59 to 64 feet, and at half that depth 

 the oscillations of the thermometer, from the influence of the 

 seasons, scarcely amount to half a degree. In tropical cli- 

 mates 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, cer- 

 tain determination of the mean temperature of the air of 

 different places.^ This mean temperature of the air at a 

 fixed point, or at a group of contiguous points on the surface, 

 is to a certain degree the fundamental element of the climate 

 and agricultural relations of a district ; but the mean tem- 

 perature of the whole surface is very different from that of 

 the globe itself. The questions so often agitated, whether the 

 mean temperature has experienced any considerable differences 

 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, 

 Bcarcely been invented more than two centuries and a half, 

 and its scientific application hardly dates back 120 years. 

 The nature and novelty of the means interpose, therefore, very 

 narrow limits to our investigation regarding the temperature 



* Bo«s3^ingault, Sur la Profondeur a laquelle se trouve la Couche d» 

 T^'i'^rature invariable entre les Tropiques, in the Annalet de Chimin 

 »e M Pkttstque, t. liii., 1833, p. 225-247. 



