7H 



THE POPULAR SCIENCE MONTHLY. 



the air about one hundred and fifty yards 

 behmd cither fore-breast. The actual tem- 

 peratures in March, 1880, after the two ex- 

 cavations had been connected, were 8V. 

 The temperature of the air immediately at 

 the two fore-breasts was brought down to 

 82 Fahr., while boring, and to 8C Fahr. 

 while clearing away the debris, or to about 

 6 below the calculated point, by the opera- 

 tion of an extra supply of compressed air. 

 The question of cooling the air in the tun- 

 nel-galleries presents great difficulties, for 

 the heat of the I'ocks 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 i-ise. Dr. 

 Stapff is not able to recommend any better 

 cooling apparatus than a combination of the 

 cooling mixture of ice and salt and quick- 

 lime. 



Belation 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 lit- 

 toral and some parts of the valley of the 

 Saone. 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 cor- 

 respond 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 Saone, of the Lower 

 Rhone, were regions in which relatively lit- 

 tle 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 bur- 

 den ; 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. De- 

 scending 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 in- 

 fluence of altitude and of the situation as 

 related to moist winds ; 3. Those which 

 were connected with the action of neighbor- 

 hood 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 expos- 

 ure to rain-bearing winds. The maxima of 

 the hills of Normandy, Brittany, and Poi- 

 tou were due to the fact that those prov- 

 inces 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 considei-a- 

 bly 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 Rhone, but little water fell in the basin 

 of the Adour. Heavy rains did not fall si- 

 multaneously 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 



