It is not only necessary to furnish pure air for the miners to breathe, but fresh air is required in great volume to cool off the rock and keep down the heat in the drifts and cross-cuts of the lower-levels. As the shafts and inclines increase in depth there is a constant and corresponding increase of heat in the rocks into which the works are advanced. At the depth of from 1,500 to 2,000 feet the rock is so hot that it is painful to the naked hand. In many places, from crevices in the rock, or from holes drilled into it, streams of boiling water gush out. In these places the thermometer often shows a temperature of from one hundred and twenty to one hundred and thirty degrees. It is as hot as in the hottest Turkish bath. In these places men could not live but for the supply of cool air that is pumped into the drift or other place in which they are at work; even then the temperature often remains as high as one hundred and ten degrees. The rock in a newly opened level retains its heat for months, however much air may be brought into the mine. Nearly all the leading mines on the Comstock are down to where the rock is exceedingly hot. The Crown Point and Belcher Companies are down 1,700 feet; Yellow-Jacket, 1,740; Bullion, 1,700; Imperial-Empire, 2,100; Gould & Curry, Best & Belcher, Consolidated Virginia, and Ophir, each 1,700; while the Savage Company are down nearly 2,300, and the Hale & Norcross, about 2,200. In the two mines last named they find it fearfully hot. As the Savage Company have started up machinery capable of sinking to the depth of 4,000 feet, they will presently be in danger of dropping into the great central fires of the earth.
As depth is attained it is found necessary to increase the size and capacity of the blowers used and the main pipes through which the air is forced into the mines have now been increased to about two feet in diameter, whereas the diameter of those first used was only about six inches. With a small pipe the air backs up on the blower and there is a waste of power. The pipe should be so large that there is no longer any perceptible back-pressure—so large that all the air blown into it finds an abundance of room in which to advance without encountering the resistance of its own elasticity. The pipes should be enlarged until the air goes through without any rebounding.
It is a question in many minds whether the miners of Nevada have gone the right way about the ventilation of their mines; whether instead of forcing air into the lower-levels they should not pump the foul and heated air out, when pure air would rush down and fill the vacuum thus created. In the mines of Germany they practice this plan of pumping out the foul air. In Nevada, however, it is not likely that it would answer so good a purpose as the plan of pumping in fresh air. By blowing in air as is now practiced there is always more or less good air at the face of a drift about the end of the pipe, but by the pumping-out plan the air surrounding the end of the pipe would be sucked into it, and that which would reach the men would be such as flowed a long distance in contact with the heated rock forming the walls of the cross-cut or drift. American miners work so fast that the rock does not have much time in which to cool behind them. Therefore the better plan for them seems to be the reverse of that practiced in the Old World.
It remains to be seen what effect the Sutro Tunnel will have in creating a circulation of air in the lower-levels of the mines when it shall have been completed. This tunnel, about which so much has been said in Congress and elsewhere, starts at the edge of the valley of the Carson River, in a southeasterly direction from Virginia[Virginia] City, and is intended to tap the Comstock lode at the depth of 200 feet. Its total length will be 20,145 feet. Work was commenced on it in October, 1859, and it has now been extended a distance of between nine and ten thousand feet. About 1,100 feet of the tunnel, from the mouth in, has been made of full size, twelve by sixteen feet; the remainder, what is called the header, is six by seven feet in size.
There are along the line of the tunnel, which runs under several mountains of considerable size, four shafts. These were designed to be sunk down to the level of the tunnel, when work on the tunnel might be prosecuted in two directions from the bottom. Shaft No. 1 is located at a distance of 4,915 feet from the mouth of the tunnel; shaft No. 2, 9,065 feet from the mouth of the tunnel; No. 3, 13,545 feet from the mouth of the tunnel; and shaft No. 4, 17,695 feet from the same point, and 2,450 feet from the point where the tunnel will intersect the Comstock lode. Shafts Nos. 1 and 2 are down to the level of the tunnel and work has been done through them. Shaft No. 1 is 523 feet, and shaft No. 2, 1,041 feet in depth. Shafts Nos. 3 and 4 are not yet down to the level of the tunnel, the “header” of which is progressing between shafts Nos. 2 and 3. When the tunnel shall have been completed, there will be a connection between the Comstock lode and shaft No. 4 through which there will be a circulation of air. This shaft (No. 4) will be 1,485 feet in depth, and when the connection is made the air will either pass down it, along the tunnel a distance of 2,450 feet and out through the mines at the point of intersection, or will pass down through the mines and out through the shaft. Which way the draught will be, no man can say, as the course of currents of air underground is governed by laws not yet well understood. Whichever way the draught may be, however, there will be a great improvement in the circulation of the air in the lower-levels of the adjacent mines, to the depth of 2,000 feet.
However diligently work may be prosecuted on the Sutro Tunnel, it must be some years yet before it can be completed to the point of intersection with the Comstock lode. Meantime there is being sunk at the distance of about 2,000 feet east of the lode, and about 450 or 500 feet west of shaft No. 4 of the Sutro Tunnel, a shaft which will be the largest and most perfect in every respect ever sunk in that country. This shaft is being sunk by a combination of three leading mining companies—the Chollar-Potosi, Savage, and Hale & Norcross. It will be ten by thirty feet in size, divided into four compartments by stout plank partitions, and the machinery placed over it will be of a capacity to sink it to the depth of one mile.
Rapid progress is being made in the sinking of this great shaft. At proper intervals drifts will be run from it to the Comstock lode. The first drift will probably be run at the depth of 2,000 feet, and it will reach the lode long before the completion of the Sutro Tunnel, and as regards ventilation, will do all that could be expected of the tunnel. As two or three of the leading mines are already working at a depth of nearly 2,500 feet, the big shaft must be looked to for ventilation everywhere below the depth of 2,000 feet; therefore below this depth drifts will doubtless be run between the lode and the shaft at frequent intervals.
Owing to the lead dipping to the east at an angle of from thirty to fifty degrees, the distance necessary to be run to connect the lode and shaft will constantly decrease until at a certain depth the shaft itself will cut the lead, after which time the drift to reach and ventilate the vein must be run to the eastward. A branch-track connects this shaft with the main Virginia and Truckee Railroad.
CHAPTER LXVII.
BELOW THE WATER-DEPOSITS.
In countries where no mining is done it is the prevalent opinion that at a certain depth the earth is full of water, and that the deeper we go the more water will abound. This is a mistaken notion. After delving beyond certain bounds, water ceases to be generally disseminated in the earth. This is after we have gone below the “scalp” or surface-water of the country. Until we have passed through this scalp, water is found almost everywhere. This being the case, it is quite natural that persons residing in countries where wells sunk in search of a supply of water are the deepest works of the kind undertaken, should imagine overwhelming floods of water to exist everywhere far down in the bowels of the earth.