pipe of leather (c) is secured to the cheek by means of a metal washer, and, pressed down by the compressed air on the end 
of the ordinary pipe, makes the joint sufficiently air-tight, although it does not hinder the advance or the retreat of the pipe. In the tunnel itself—where the temperature is not subject to such fluctuations—these precautions are not necessary, and the pipes are carried along the walls, supported by brackets, as far as the end of the finished work. Through these pipes highly compressed air is conducted, and is delivered at the end of the "advanced gallery" where the boring-machines are at work, with only a slight diminution in its pressure, notwithstanding the escapes which occur at the joints.
On entering the tunnel one is struck by its size. The Italians, with a magnificent disregard of expense, or from regard to the future, have constructed it not only with two pairs of rails,[1] but with a footpath on each side. From the rails to the crown of the arch its height is just 20 feet, and its width is 26 feet 6 inches. The next thing that is noticed is that it is almost everywhere lined with masonry; a small fraction only of the rock is left unsupported. The stone that is used is not obtained from the tunnel itself, but is quarried several miles away, near to St. Michel.[2] Not observed,
