77 



so that at a time <>l' thaw the snow-water is yielded up more 

 gradually. Nor must the fact he overlooked that when snow in a 

 forest melts, the ground absorbs the water to a much greater extent 

 than happens in the open country. In the latter case tin; ground is 

 probably frost bound, so that the snow-water cannnot be absorbed 

 hy the soil, whereas forest soil, being protected by, trees, never 

 freezes to the same extent, and is consequently in a better position 

 to absorb snow-water. The result is that not only does a forest miti- 

 gate the violence of floods, but the snow water that Mows from its area 

 is less muddy than would otherwise be the case. 



Forests not only affect the degree of moisture in soil, but they also 

 •exert considerable influence on the soil temperature. Although this 

 influence is greatest at the surface of the ground, it is also perceptible 

 to a depth of several feet. On the average of a Large number of con- 

 tinental stations it was found that woods of various species and ages 

 depressed the mean annual temperature at the surface of the ground 

 hy about 2*6° F., while even at the depth of four feel the reduction 

 of temperature was 2°. 



This general cooling influence is due to a variety of causes. The 

 foliage of the trees excludes the sun's rays, the decaying vegetable 

 matter that covers the ground prevents the free exchange of air 

 between the soil and the atmosphere, while the water in the soil 

 absorbs much heat without its temperature being much affected. 



While woods have a depressing influence on the mean annual 

 temperature, it is found that this effect is much greater in summer 

 than in winter. On the average of 11 German stations the July tem- 

 perature of the surface soil in the forest was found to be 7° F. lower 

 than that in the open field, wdiereas in December the former was rather 

 warmer than the latter. Forests, therefore, tend to equalise the tem- 

 perature of water collected in them, the temperature being slightly 

 raised in winter, and markedly reduced in summer. This result 

 would appear to be of considerable practical and hygienic importance 

 where a supply of water for domestic purposes is concerned. 



To the credit of forests is also to be placed the fact that they 

 exercise a purifying influence both on the air and on the soil, germs 

 of all kinds being markedly scarcer in a well-wooded district than in 

 a similar extent of tree-less country. — Board of Agriculture. 



4, Whitehall Place, London, S.W.. 

 January, 1904. 



618. AGRICULTURAL GEOLOGY. 



THERE is a considerable want of knowledge of the elements ot 

 Geology, especially the formation of the superficial crust of the earth, 

 among many of the followers of agriculture in the tropics. So far as 

 our knowledge goes there is no book which treats specially on the 

 soils of tropica] countries, but there has recently been published by 

 Methuen & Co. (1903) a very valuable treatise of MIS pages for the 

 use of candidates lor the International Diploma of Agriculture which 

 appears to be well suited for the guidance of students in all countries. 

 The author is J. E. Marr. M.A., F.R.S.. Fellow of St. John's College. 

 Cambridge, and its price is within the means of the humblest. 



