Page:Popular Science Monthly Volume 7.djvu/220

208 by other unknown conditions which may in a greater or less degree change the direction or the velocity of the rain-current, or the point and degree of condensation of the watery vapor contained in it.

M. Dausse, in a memoir which appeared in the "Annales des Fonts et Chaussées," uses the following argument: "Rain is formed when a warm and humid wind comes in contact with strata of cold air; and since the air of forests is colder and more humid than that of the open, rain must fall there in greater abundance."

To gauge experimentally the influence of forests on the rainfall of a district, or, in other words, to ascertain the condensing power of forests, we have compared the results obtained in observations made: 1. Above the forest; and 2. At the same altitude, and at so small a distance from the forest, that any observable difference could be attributed only to the influence of the latter.

"We now made the following observations in the heart of the forest of Halatte, which embraces 5,000 hectares of land. At the height of about six metres (say 20 feet) above a group of oaks and hornbeams eight or nine metres high, we placed a pluviometer, a psychrometer, maximum and minimum thermometers, and an evaporometer, so as to ascertain at that point the amount of rainfall, the degree of saturation of the air, and the rate of temperature and of evaporation.

In the open air, at the distance of only 300 metres from the forest, and at the same height above the ground as in the former case, we placed similar instruments under the same conditions. With regard to the rainfall and the degree of saturation, we give a summary of the first six months' observations, as follows: