but nevertheless existing, is a covered way about 3 feet 4 inches high, and 4 feet wide, which is made in the floor of the tunnel 
between the rails; it is in fact a tunnel within a tunnel. Originally its dimensions were less, and it was intended merely as a subway in which the pipes conveying the compressed air might be placed, and as a drain; it was found convenient to enlarge its size, and since that has been done, it has—on at least one occasion—served a purpose for which it was not originally intended. On the 15th of September 1863, a sudden fall of rock occurred, which killed several miners and imprisoned about sixty others who were at work in the advanced gallery. They were greatly alarmed, and expected to be starved; but at last one of them remembered this subway, and they escaped by its means. Since that time the miners, knowing they have this exit, have troubled themselves very little about éboulements.
The temperature of the tunnel remains tolerably uniform throughout the year, but it is much higher in some parts than it is in others. On the occasion of my last visit, the exterior temperature was 63½° Fahr. in the shade; a mile from the entrance it was 65°, and the mouth looked like the sun on a misty November day. At two miles the thermometer showed 70°, the atmosphere had become foul, and the mouth was invisible. In two hundred and fifty paces more, it had risen to 75°, the tunnel was filled with dense
