The Romans should doubtless rank as the greatest tunnel builders of antiquity, in the number, magnitude, and useful character of their works, and in the improvements which they devised in the methods of tunnel building. They introduced fire as an agent for hastening the breaking down of the rock, and also developed the familiar principle of prosecuting the work at several points at once by means of shafts. In their use of fire the Romans simply took practical advantage of the familiar fact that when a heated rock is suddenly cooled it cracks and breaks so that its excavation becomes comparatively easy. Their method of operation was simply to build large fires in front of the rock to be broken down, and when it had reached a high temperature to cool it suddenly by throwing water upon the hot surface. The Romans were also aware that vinegar affected calcareous rock, and in excavating tunnels through this material it was a common practice with them to substitute vinegar for water as the cooling agent, and thus to attack the rock both chemically and mechanically. It is hardly necessary to say that this method of excavation was very severe on the workmen because of the heat and foul gases generated. This was, however, a matter of small concern to the builders, since the work was usually performed by slaves and prisoners of war, who perished by thousands. To be sentenced to labor on Roman tunnel works was thus one of the severest penalties to which a slave or prisoner could be condemned. They were places of suffering and death as are to-day the Spanish mercury mines. Besides their use of fire as an excavating agent, the Romans possessed a very perfect knowledge of the use of vertical shafts in order to prosecute the excavation at several different points simultaneously. Pliny is authority[1] for the statement that in the excavation of the tunnel for the drainage of Lake Fucino forty shafts and a number of inclined galleries were sunk along its length of 3¹⁄₂ miles, some of the shafts being 400 ft. in depth. The spoil was hoisted out of these shafts in copper pails of about ten gallons’ capacity by windlasses. [1] “Tunneling,” Encly. Brit., 1889, vol. xxiii., p. 623.