OBSERVATIONS BY EBBRMATER. 207 



and in sharpening stUl more the extremes of heat and cold. But 

 these thermal efifeots are not alone confined to the atmosphere, but 

 have, as well, a particular influence upon the soil. The Bavarian 

 investigations have clearly demonstrated that the average annual 

 temperatures at various depths beneath the surface of the ground are 

 nearly equal ; that the average temperature diminishes downward 

 quite slowly and by small degrees, (at a depth of 4 feet never more 

 than 1"12°,) but that the average annual temperature of woodland 

 soil, at all depths, is below that of unwooded land, on an average of 

 3-37°. 



" Further, it has been demonstrated that the temperature of the 

 soil in the spring and summer is highest at the surface and diminishes 

 downward. In spring the difierence between the warmth of the soil 

 at the surface and at a depth of i feet amounts within the forest to 

 5 '62°. In summer these differences show 7 '50° both in the wood and 

 in the open land. But the soil of the woods in summer is 75.0? 

 colder than that of the unwooded land, and the greatest difference 

 has been found at a depth of 2 feet. 



" In contrast to spring and summer, the temperature of the soil 

 increases in autumn and winter, from the surface to a depth of 4 feet 

 beneath. In autumn this advance of temperature, according to the 

 Bavarian observations, amounted to 4'50° outside of the forest and to 

 3"49° within the forest ; in winter, relatively 4'21'' outside and 4-41* 

 inside the woods. Wooded and unwooded lands have, therefore, in 

 winter, to the depth of 4 feet, very nearly the same temperature ; the 

 result of which is that the influence of the woods upon the temperature 

 of the soil is not less than that which they exert upon the tempera- 

 ture of the atmosphere.* 



* It cannot be supposed that trees have any vital process by which a degree of heat 

 is maintained above the medium in which they grow. Their trunks, branches, and 

 leaves are heated and cooled in the same manner as inorganic bodies under like condi- 

 tions of exposure. Bat wherever evaporation or condensation is taking place, the 

 same change occurs in them as elsewhere, and the universal law of thermal result ap- 

 plies. " The trunks of trees," says M. Decquerel, " only acquire their maximum tem- 

 perature after sunset. In summer it occurs as late as 9 p.m,, while in the air the max- 

 imum temperature occurs from 2 to 3 p.m. Changes of temperature take place very 

 slowly in the tree, but in the air they are rapid. When the leaves are cooled by noc- 

 turnal radiation they recover from the trunk by radiation the heat they have lost. 

 The temperature of the air above the trees which have been heated by solar radiation 

 acts on the temperature of the air, and prevents it falling as low as it would otherwise 

 have done." — Comptes Bendus, Seances de I'Acad. des Sdences, May 22, 1855, tome ix, 

 p. 1049 ; also Jour, of Scottish Meteorological Soc., (new ser.,) i, 234. [I give the note 

 as given appended to the text, but I do not homologate what is said in the opening 

 sentence.— J. C. B.] 



