262 RESULTS OF COMPARATIVE OBSERVATIONS: FRANCE. 



FRANCE. 



In April, ISGG, meteorological observations were commenced by Pro- 

 fessor Matbieu, of Nancy, at two forest stations and at an agricultural 

 station, with the view of determining facts concerning evaporation, rain- 

 fall, &c., and the influence which a wooded or unwooded country has 

 upon the moisture received from the air.^ 



In an article ])ublished in 1874, eight years after these observations 

 were commeuced,^ Professor Matbieu remarks that the results com- 

 pletely confirm those made at Asscbaffenburg : 



Tbese observations have shown, as to that station, that the mean temperature of tlie 

 air at 1.5 meters above the soil is at all seasons, and especially in summer, lower within 

 the woods than in the fields, and consequently that the mean annual temperature of 

 the air in forests is below that of the open country. The difference, however, is not 

 large, and in 1872 did not exceed 0o.51 of the centigrade scale. At Asscbaffenburg, 

 the results of several years' observation make the difference 0°.78. 



The results of evaporation are not less remarkably coincident. The 

 Trench report of evaporation from an open surface of water, within and 

 without the woods, in 1873, showed the proportion as 37 per cent. — the 

 German station as 3G. The evaporation from a free water surface is the 

 same as from a surface of native soil saturated. A covering of dead 

 leaves has the same protecting influence on the soil as dense growth of 

 trees. 



The results near Nancy still further agree in showing, that woodlands 

 exert a much greater influence upon the evaporation than upon the tem- 

 perature of the air and the soil, which leads to the conclusion that evap- 

 oration and humidity are greatly influenced by the greater or less agita- 

 tion of the air. Hence the importance of a covering of leaves, and of 

 screens to check the action of the winds. 



It is further noticed that a wooded country tends to increase the pre- 

 cipitation of atmospheric moisture, most sensibly in sloping and mount- 

 ainous regions, and that on plains this effect is scarcely appreciable ; 

 and, furthermore, that about 26 per cent, of the rain is intercepted by the 

 foliage of the trees. The amount diflers much between forests of difler- 

 ent kinds, and in beech GO years old it was found to be but 17 per cent. 

 Here again the observations at Nancy confirm the principle, although 



' The stations were located as follows: 



Cinq-Travches; on a plateau 380 meters above sea-level, in the midst of the vast for 

 est of La Haye, which crosses the chain of Jurassic hills of lower oolite traversing the 

 department of Meurthe from north to south. Two udometers were placed to measure 

 the amount of water that fell under the different conditions there found. The first 

 was placed in a forest (lightly cleared) of hornbeam and beech 41 years old, and con- 

 sisted of a receiver through which passed the trunk of a tree of average size, and 

 with a surface equal to that covered by the tree-top. It showed exactly the tiepth of 

 water which reached the ground, either from the leaves or their intervals, or from the 

 trunk, with whatever irregularity it might be sifted by the covering. The other 

 udometer was placed about 200 meters from the former, in the middle of a cleared 

 place several hectares in extent, and at a distance from every object that could hinder 

 or modify the rain-fall. It was of the ordinary construction. 



Belle-Fontaine; 240 meters above sea-level, at the bottom of a valley open from south 

 cast to uorthwest, on the border of the forest plateau of La Haye, at only 2 kilometers 

 from Meurthe. The woods are like those of the former station, 71 years old. Two 

 instruments for measuring the evaporation were placed here, exactly comparable as to 

 size and mode of consTruction, and oOO meters apart ; one in the open fields, and the 

 other in the woods. Rain-gauges were also placed, of the usual form. The evapo- 

 rating basins were filled to an equal depth, and at the end of a mouth were measured 

 to determine the changes, deductions being made for rain-fall. 



Amance, an agricultural station; 10 kilometers east of Cinq-Tranches, and of equal 

 altitude, remote from forests of any notable size. Its rain-gauge was of the usual form. 



^ Rtvue des Eaux et Forets, May, 1S74, p. 1G5. See also the Atlas Mettorologique of the 

 Observatory of Paris, where the annual results are published. 



