goes on, successive portions of air are drawn through a, and forced through the valve, b, into the receiver. If another closed vessel, at C, be connected with the lateral tube at a, it will be exhausted of air by the same process. Hence this instrument may be used to transfer gases from one vessel to another.

188. Applications of Condensed Air.-In the improved method of transmitting messages by pneumatic tubes, condensers as well as air-pumps are used; the air being forced into the tubes behind the pistons at the same time that it is exhausted in front.

In laying the foundations of bridges, and in various submarine operations, air is forced into large tubes, open at the bottom, which are sunk in the water where work is to be done. These tubes are so arranged that men can enter them and work at the bottom while the water is kept out by the pressure of condensed air.

It is possible for inen to remain for a considerable time, without injury, in an atmosphere of three or four times the ordinary density; the only inconvenience being a painful sense of oppression in the ears. This feeling takes place only at the beginning and end of the operations, disappearing when an equilibrium is established between the tension of the air in the internal ear and that without.

189. Application to Tunnelling and Mining. - One of the most important uses of compressed air has been in the boring of tunnels through mountains of solid rock.

The excavation of the Mont Cenis Tunnel in the Alps, and of the Hoosac Tunnel in Massachusetts, was accomplished by means of machines driven by compressed air.

At the Hoosac Tunnel, the water-power of the Deerfield River, which flows near the eastern entrance, was used to operate several powerful compression-pumps, which forced the condensed air through an iron pipe to the point of working in the tunnel.

Here it was made to drive a number of drilling-machines, by which the rock was perforated. The machines consisted

essentially of cylinders fitted with pistons, to which the drills were attached. Eight or ten of these machines were fastened to a heavy iron framework resting on wheels, by which it could be moved forward and back, on rails laid for the purpose (Fig. 134).

When in use, this framework was brought up and firmly fastened near the "heading" to be operated on. The drilling-machines were

then fixed in the proper position on the framework, and the compressed air from the iron pipe was conducted to the several machines by smaller flexible tubes. Here it was admitted to the cylinders, alternately before and behind the pistons which carried the drills, driving them with great force and rapidity against the rock.

Fig. 135 represents the iron framework, or carriage, with four drills attached. The flexible tubes shown in the figure carry the air to the machines from the iron pipe laid along the bottom of the tunnel.

Fig. 136 represents a form of the drilling-machine which is now extensively used in mining operations. It is mounted upon a column, on which it may be raised or lowered by means of the screw-thread cut upon its surface. It is also arranged so that the drill may be driven in any direction required.

190. Advantages in the Use of Compressed Air. For work in tunnels, deep mines, and other confined spaces, there are several advantages in the use of compressed air:

1. The power may be transmitted through a great distance with very slight loss.

At the Hoosac Tunnel, when the work was done at a distance of nearly three miles from the compressors, the loss of power was less than four per cent of the whole.

2. The air, after doing its work in the machines, escapes and serves as a fresh supply of pure air, and drives out the

smoke and the noxious gases which would otherwise accumulate from the blasting, the burning of lamps, and the breathing of the workmen.

3. In deep mines, where the heat is often oppressive, the expansion of the air, as it escapes, lowers the temperature.

ward, in the form of a jet, by the tension of compressed air. Hero's Fountain, one form of which is shown in Fig. 137, is operated in this way.

It consists of two globes of glass, connected by two metallic tubes. The upper globe is surmounted by a brass basin, connected with the globe by tubes, as shown in the figure.

To use the instrument, the tube which forms the jet is withdrawn, and through the opening thus made, the upper globe is nearly filled with water, the lower one containing air only. The jet tube is then replaced, and some water is poured into the basin.

The water in the basin, acting by its weight, flows into the lower globe, through the tube shown on the left of the figure, as indicated by the arrow-head. This flow of water into the lower globe forces out a part of the air in it, which, ascending by the tube shown on the right of the figure, accumulates in the upper globe. The pressure of the air in the upper globe, acting upon the water in that part of the instrument, forces a part of it up through the jet tube, giving rise to a jet of water, which may be made to play for several hours without refilling the instrument.

192. The Atmospheric Inkstand. This form of inkstand now in common use illustrates the principles of atmospheric pressure.

It is

It is represented partially filled with ink in Fig. 138. The body of the inkstand is air-tight. Near the bottom is a tube for supplying the ink as wanted, and also for filling the inkstand when necessary. filled by turning it until the tube is at the top, when the ink can be poured in through the tube. The pressure of the atmosphere prevents the ink from flowing out. When the ink has been used till its level falls below o, where the tube joins the main body of the inkstand, a bubble of air enters, and rising to the top, acts by its pressure to fill the tube again, and so on until the ink is exhausted.

Summary.

Measure of the Elastic Force of Gases.

Fig. 138.

Mariotte's Law.

Verification of the Law.