368 



APPENDIX 



quantity of water. A similar fact had already been observed in 1878 and 1890 in the 

 reports published by Mathieu and M. Bartet ^ on the results obtained in those years by 

 the "Station de recherches" of Nancy. 



This is the more instructive, inasmuch as it is the winter rains which are the most 

 important from the point of view of the feeding of springs. 



The fact is very naturally explained by the condensation of aqueous vapor on the 

 considerable surface presented by the crests of forest trees.^ 



At all seasons of the year and all hours of the day the trees are, in fact, colder than 

 the surrounding atmosphere. Nevertheless the difference is greater in summer than 

 in winter, and during the day rather than at night. It is also greater at the base of the 

 trunk than among the branches. 



Here are some figures on the subject. The Swiss observations give results recorded 

 on an average of 12 years; those in Bavaria were undertaken during two years only: 



DIFFERENCES (IN CENTIGRADE DEGREES) BETWEEN THE TEM- 

 PERATURE OF THE TREES * AND SURROUNDING AIR 



Winter 



Larch (near Interlaken) 



Spruce (near Berne) 



Beech (near Porrentruy) 



Miscellaneous species (Bavaria) | 



0.4 

 0.9 

 0.3 

 1.3 



1.4 



* In the Swiss ejcperiments the temperature of the tree was taken at breast-height, 

 while in Bavaria it was in the branches of the crown. 



These reports suffice to give us the key to the important phenomenon noted above. 

 Trees with considerably spreading crowns form very excellent condensers of vapor 

 from atmospheric water which they conduct to the soil in a Uquid condition; this process 

 is naturally more marked at the beginning and end of winter, periods when the atmos- 

 phere of our latitudes, especially in forests, is very near the point of saturation. The 

 slightest lowering of the temperature is then sufficient to produce condensation. 



In conclusion, the results obtained by the observations conducted at Cinq-Tranch6es 

 can be summed up in the following manner: Of a hundred millimeters (3.94 inches) of 

 atmospheric water, the instrument placed under shelter of the crest of a young horn- 

 beam only received in an average year 92 m.m. 4 (3.64 inches); therefore 7 m.m. 6 

 (0.299 inch) were retained by the crown a balance produced by the amount of condensa- 

 tion from branches and the top of the trunk. 



'^ " M6t6orologie Compar(5e, Agricole et Forestilre. . . . Observations faites k 

 la Station de Recherches de I'Ecole National Forestifere," published by M. E. Bartet, 

 Paris, 1890 ("Bulletin du Ministfere de I'Agriculture"). 



^ It happens sometimes that a considerable quantity of water is supplied to the 

 forest soil m a very short period of time. This occurred in January, 1882. The east 

 wind accompanied by fog had deposited such a quantity of hoar frost on the trees that 

 a great number of branches were broken by the weight of it. This first took the form of 

 icicles ten centimeters in length. A twig covered with them, cut off with great care, 

 weighed 550 grams; relieved of its burden, it only weighed 70 grams ("Bulletin de la 

 Commission M6t6orologique de Meurthe-et-Moselle"). This frost in melting on January 

 17 was equal to a rainfall of 7. m.m. 4 according to the rain-gauge under the trees at 

 Cinq-Tranch6es. In January, 1879, a branch of birch from the forest of Fontainebleau, 

 weighed by M. Croizette-Desnoyers, covered with a thick crust of rime, turned the 

 scales at 700 grams. Weighed again after the frost had melted, the result given was 

 only 50 grams. 



