Page:Popular Science Monthly Volume 18.djvu/732

714 the air about one hundred and fifty yards behind either fore-breast. The actual temperatures in March, 1880, after the two excavations had been connected, were 87°. The temperature of the air immediately at the two fore-breasts was brought down to 82° Fahr., while boring, and to 86° Fahr. while clearing away the débris, or to about 5° below the calculated point, by the operation of an extra supply of compressed air. The question of cooling the air in the tunnel-galleries presents great difficulties, for the heat of the rocks is inexhaustible, and the air, no matter in what condition it may be delivered, becomes heated up again nearly as soon as it is distributed. The use of jets of water is objectionable on account of the increase of dampness that attends it, and the mists to which it gives rise. Dr. Stapff is not able to recommend any better cooling apparatus than a combination of the cooling mixture of ice and salt and quicklime.

Relation of Elevation and Exposure to Rainfall.—M. Th. Moureaux has drawn up a set of maps based upon the reports of the Central Meteorological Bureau of France, which show what was the distribution of rain over the country for each month of the year, and for the whole year, 1878. Except in February and September, which were dry, the year was a moist one; the rains were excessive, except in the Mediterranean littoral and some parts of the valley of the Saône. The amount of rain increased with the height of the locality. The map shows at the first glance that the low regions, the plains, correspond with the smallest falls. The minima were constant during the whole year; in constructing the monthly maps, the absolute minimum in each month was found to be on the littoral of the Mediterranean, and the relative minima were found to correspond to the large valleys, whatever might be their direction. The valley of the Loire below Orleans, and those of its affluents on the left bank, the basin of Paris, the valleys of the Garonne, of the Saône, of the Lower Rhône, were regions in which relatively little water fell. In mountainous regions, at the same height, the rains were much more abundant on the slope exposed to the direct action of moist winds than on the opposite slope. When a mass of air rose along the side of a mountain, it became steadily cooled, its load of moisture was relatively increased, and the clouds soon precipitated their burden; the condensation was more active as the difference of temperature increased, and as the air of the plain approached the point of saturation. The inverse phenomenon was produced on the opposite slope. Descending the side opposed to the direction of the wind, the air became warmer, and its temperature further and further from its dew-point; the rain was light and often there was none. The minima were thus constant during the several seasons. The same was not the case with the maxima. They could be divided into three groups: 1. Those maxima wholly due to altitude; 2. Those which were attributed to the combined influence of altitude and of the situation as related to moist winds; 3. Those which were connected with the action of neighborhood to the sea. In the first group, the rule was absolute; the highest points constantly received more rain than the surrounding places of a loss altitude. But this was not the case with the maxima which were due to the neighborhood of the sea or to exposure to rain-bearing winds. The maxima of the hills of Normandy, Brittany, and Poitou were due to the fact that those provinces lay near and to the east of a great mass of water. This influence was made effective by the frequency of winds from the west, which drove toward those regions the moist air of the ocean; accordingly, it was most clearly manifested during the cold season; but the maxima were considerably lessened, or disappeared when the rains came from the southeast. So the maximum of the gulf of Gascony resulted from the predominance of winds from the west or northwest; and, when the south winds were pouring torrential rains into the basin of the Rhône, but little water fell in the basin of the Adour. Heavy rains did not fall simultaneously over the whole of the central plateau. They were limited to the slopes exposed to the direct action of rain-bearing winds. When brought by winds from the south or southwest, as was most frequently the case, they fell upon the side toward the ocean; while, when they came from the south and southeast, they were deposited on the