connection with the recipient being closed by the compressed air imprisoned in the vessel. The aereomotori continue their motion, another revolution of the main shaft shuts the exhaust and opens the equilibrium or admission valve; the column of water is again permitted to act, and the same action is repeated, more air being forced into the recipient at each round or _pulsation_ of the machine. Now, supposing no consumption of the compressed air to take place beyond that used for driving the aereomotori, it seems evident that the water in the vessels would be gradually forced out, owing to the growing pressure of the air inside, above the pressure of the column of water coming from the higher cistern; but the communication with this higher cistern is always kept open, the column of water acting, in fact, as a sort of moderator or governor to the compressing-machine, rising or falling according to the consumption of the compressed air, and always insuring that there shall be a pressure of six atmospheres acting against the valve at the summit of the vertical column. A water-tube placed on the outside of each group of recipients, with a graduated scale marked on it, indicates at a glance the consumption of air. If the perforating-machines in the tunnel cease working, the pressure augments in the recipients, and the water in them falls until an equilibrium is established, {68} between the pressure of the column of water and the force of the compressors, until, in fact, these work without being able to lift the valve at the summit of the vertical compressing column. On the other hand, if more air than usual be used for ventilating the tunnel, or by an accidental leakage in the conducting-pipes, the water rises rapidly in the recipients, and consequently in the water-gauge outside, and in thus creating an equilibrium, indicates the state of things. By this means a continual compensation of pressure is kept up, which prevents any shock on the valves, and causes the machine to work with the regularity and uniformity of a steam-engine provided with a governor. In every turn of the main shaft, a complete circle of effects take place in the compressors; and experience has shown that three turns a minute of the shaft--that is, three _pulsations_ of the compressing-machine per minute--are sufficient. It will thus be seen that a column of water, having the great velocity due to a head of eighty-four feet six inches, acts upon a column of air contained in a vertical tube; the effect of this velocity being to inject, as it were, a certain quantity of air into a recipient at each upward stroke of the column, and at each downward stroke drawing in after it an equivalent quantity of atmospheric air as a fresh supply. The ten recipients charged with air compressed to six atmospheres (ninety pounds on the square inch) in the manner above explained, serve as a reservoir of the force required for working the boring-engines in the tunnel, and for