Let us now examine the end and action of this machinery. As the aereomotori which work the valves of the machines for forcing air into the recipients are themselves worked by compressed air coming from the recipients, it is evident that before we can put the compressing-machines in motion, we must have already some supply of compressed air in the {67} cylindrical vessels. This supply of air, compressed to a pressure of six atmospheres, is obtained in the following manner: Each group of five recipients, filled with air at the ordinary atmospheric pressure, is put in communication with a large pipe which enters into a cistern placed in the side of the mountain at about one hundred and sixty-two feet above the floor of the compressing-room. The first operation, then, is to open the equilibrium valves placed at the bottom of the two pipes (one from each group of recipients); water then rushes into the vessels, compressing the ordinary air therein contained to about a pressure of six atmospheres. A communication is now opened between this compressed air and the cylinders of the aereomotori, which commence their action precisely as a steam-engine would do on the admission of steam; a rotary motion is given to the main shaft; and the equilibrium valves, placed in chambers at the bottom of each of the ten pipes coming from the cistern of water placed in the house above, are opened. We will observe the operation in one of the ten lines of action, as it were, consisting of the pipe conducting the water from the cistern, the compressing-machine, and the cylindrical recipient. The equilibrium valve at the bottom of the pipe being opened in the manner above explained, the water, with its head of eighty-four feet six inches, rushes past it, along a short length of horizontal pipe (in which is an exhaust valve, now closed), and begins to mount a vertical column or tube of cast-iron about ten feet high and two feet in diameter: the air in this column undergoes compression until it has reached a pressure sufficient to force open a valve in a pipe issuing from the summit of the tube, and connecting it with the recipient. This valve being already weighted with the pressure of the air compressed to six atmospheres by the means previously explained, a certain quantity of air is thus forced into the vessel; at this moment, another revolution of the main shaft causes the equilibrium valve at the bottom of the conducting-pipe to be shut, and at the same time opens the exhaust valve at the foot of the vertical column. The head of water being now cut off, and the exhaust open, the water in the vertical column begins to sink by its own gravity, leaving a vacuum behind it, if it were not for a small clack-valve opening inward in the upper part of the compressing column, which opens by the external pressure of the air, so that by the time all the water has passed out of the exhaust valve, the compressor is again full of atmospheric air; the valve in