Page:Popular Science Monthly Volume 18.djvu/731

Rh and land-slides, which give the place a curious aspect. The mines belong to the Government, as do also the salt-mines, and are farmed out for a small sum. They are not very actively worked, and the product is small. The process for extracting the gems is much like that pursued in mining for the sale, except that, instead of using a ball of clay, the miners burn a bunch of dry grass in the hole, taking precaution, as soon as the cracks appear, not to damage the turquoises which may be incased in the block. The stones are generally found in groups, often numbering twenty-five or thirty, incrusted with a thin calcareous envelope which is white next to the mineral, brown on the side next to the rock. The khaki, or earthy turquoises, are found in the valley adjoining the hills, in a soil composed of gravel and rounded stones resting on a clay subsoil. After the earth has passed through two or three washings, a considerable number of turquoises are left, of moderate size, but pale and of little value, if the diggings are fresh. The turquoises in the older pits have a better color, because, the miners say, the stones acquire their color with age. Among the largest turquoises which have been mentioned are one of which a drinking-cup was made for the Shah of Persia, and one in which the treasure of Venice was kept, and which weighed several pounds. Generally the large turquoises are pale or discolored, and of little value, and are used principally for the decoration of furniture, and of the saddles and bridles of rich Persians.

Heat in Tunnel-Excavations.—Dr. F. M. Stapff, engineering geologist of the St. Gothard Tunnel, has published, in the "Revue Universelle des Mines," the results of the studies he made during the progress of the operations in the tunnel as to the highest temperature at which men can work underground, and the depth below the surface at which that temperature is likely to be met in tunneling. The limit of temperature at which men can work depends upon the length of their exposure, the amount of exertion they put forth, their condition, and the nature of the atmosphere, particularly as to its degree of moisture. It is certain that men can not become used to stand, for any length of time, a higher degree of temperature than from 140° to 165° Fahr., even when they keep perfectly still, and are in quite pure air. Men have worked at 104° on railways in the United States and Mexico, at 72° to 94° in Belgian collieries, at 125°, under exceptionally favorable conditions, in the Fahlun copper-mine in Sweden, and are said to work occasionally in the stoke-holes of tropical steamers at 156°. The highest temperature observed in the Mont Cenis Tunnel was 86°. In the St. Gothard Tunnel work was carried on at 87° on the Airolo side, in an atmosphere saturated with moisture, and at 84° on the Göschenen side, in an atmosphere less highly impregnated. Professor Du Bois-Reymond estimates that men can stand a temperature of 122° when the air is as dry as possible, but that even 104° is likely to be fatal in an atmosphere saturated with moisture; and he recommends quick lime, notwithstanding the heat it gives off, as preferable for counteracting the heat, because it absorbs the moisture, to ice, which adds to it. Salt and ice are, however, good. The highest limit of air-temperature theoretically possible in tunnel-work would be that which would induce fever-heat, or 107° in the body; the highest practicable, but still a dangerous, temperature should not raise the heat of the body over 104°. On this basis an extreme temperature of 114° would be admissible at the Göschenen end, and of 100° at the Airolo end, of the St. Gothard Tunnel. The temperature within the borings of the St. Gothard Tunnel was found to increase with the depth of the excavation, at a general average rate of 1° Fahr. per 88·1 feet of vertical depth below the surface of the mountain. The rate is subject to local variations, giving sometimes as much as 9° of error, arising from irregularities in the surface of the mountain. Thus the actual temperature is higher than the calculated temperature under depressions of the surface, and lower under peaks; but for considerable lengths of tunnel the calculated and actual temperatures substantially agree. Dr. Stapff estimated, when the excavations at St. Gothard had been driven to within about one thousand yards of the middle of the tunnel, that the temperature at the middle, before piercing the wall between the two excavations, would be 89° for the rock, and the same for