316 M. BECQUEREL, ON THE CLIMATIC EFFECTS OF FORESTS. 



The calcareous and silicioas sands are therefore the substances that 

 have the least afifinity for water, while humus has the greatest. The 

 state of division in like manner has its influence, as we see in fine cal- 

 careous soil. 



We cannot separate in this actual case the property of absorption 

 from that of aptitude for drying, which we must take into account in 

 evaporation. Experience proves that 100 parts of water in saturated 

 soil lost in four hours, at 130.75 of temperature (c), the following propor- 

 tions : 



Clayey soil 34.9 



Calcareous soil, finely powdered... 28.6 

 Humus 20.15 



Silicions sand 88,0 



Calcareous sand 75. 9 



Barren clay 52. 



Rich clay 45.7 



We see, therefore, that silicions sand is a substance that allows the 

 water to escape most easily, while humus is one that retains it for the 

 longest time. Calcareous sand loses water more easily than silicions 

 sand. 



We will further mention the results obtained by experiments of Mel- 

 loni, relative to the cooling that results in certain substances exposed to 

 nocturnal radiation, and which should be taken into the account : 



Substances. Relation in cooling effect. 



Plants of close leaves , 10.3 



Silicious sands 103 



Vegetable soil 92 



But the absorbing power being equal to that of emission, we should 

 admit that substances within the same time should warm in the same 

 ratio. Such are the elements that enter into the solution of the ques- 

 tion, or rather the questions which M. Gay-Lussac proposes. 



When the rain falls upon the soil, the upper strata begin by becoming 

 saturated. Then the excess of water passes to the next lower strata, 

 and they also become saturated, and this continues until the excess 

 above has fully saturated the parts of the soil below. 



When the upper bed dries from the evaporation into the air, it re- 

 takes from that below what it has lost, and this from the next below 

 until all the water originally absorbed is dissipated. 



As for the evaporation, it is manifestly less, all things being equal, 

 on wooded soil, than on a soil covered with turf. On the other hand 

 the Count de Gasparin,i who has made some experiments upon this sub- 

 ject, has found, in comparing the evaporation of a surface of water with 

 that of a surface of soil completely saturated, in August, and at a mean 

 temperature of 23 to 26 degrees, the following relations one to the 

 other : 



Evaporation Evaporation 

 from water. from soiL 



Fifthdav 11.7 1.3 



Sixth day 11.0 1.2 



Seventh day 9.4 1.3 



Evaporation Evaporation 

 from water. from soil 



First day 15.0 4.1 



Second day 13.7 2.5 



Third day 11.5 1.8 



Fourth day 12.0 1.3 



The evaporation, therefore, goes on rapidly at first from the soil, and 

 then becomes very slow. 



The series of experiments that we report, show that the evaporation 

 should vary considerably according to the nature and physical condition 

 of the soil, a consideration to which we have not had regard. Thus 

 soils covered with low vegetation or with woods, and in which the soil 

 is composed of humus, mingled with sand and lime or clay, absorb more 



1 Cours d' Agriculture ii, 114. 



