through BD the pressures on the sides balance each other except at D, at which part of the tube there is an uncompensated pressure on the side opposite the orifice, the effect of which is to turn the tube CD round. The

E B

same effect is produced by the water issuing at E, and a continued rotation of the instrument is thus produced. By means of a toothed wheel at A the instrument may be employed in communicating and maintaining motion in other machines.

The Piezometer.

116. This is an instrument for measuring the compressibility of liquids.

A thermometer tube CD, open at the end C, is enclosed in a strong glass vessel, which also contains a condenser-gauge EF. (See Art. 111.)

The liquid to be examined is poured into CD, and a drop of mercury is then introduced into CD so as to isolate the liquid, and the vessel is filled with water and closed by a piston. The piston A is moveable in the neck of the vessel, and, by means of a screw B, any required pressure can be produced. The gauge EF measures the pressures, and the compression of the liquid is obtained by observing the space through which the drop of mercury P is forced.

B

Q

The area of a section of CD and the volume of the bulb are found by weighing the quantities of mercury contained by the bulb and a portion of the tube.

The Hydraulic Ram.

117. The fall of water from a small height produces a momentum which by means of the Hydraulic Ram* is utilized and made to produce the ascent of a column of water to a much greater height.

K

H

B

The figure is a vertical section of the machine, AB being the descending and FG the ascending column of water, which is supplied from a reservoir at A. E is an air-vessel with a valve at C, opening upwards; at D is a valve opening downwards, and H is a small auxiliary airvessel with a valve K opening inwards.

The action of the Machine. The valve D will at first be open in its lowest position, and if water descend from A, a portion will flow through D, but the action on the valve will soon close it, and the sudden check thus produced increases the pressure; the valve C is lifted and water flows into the vessel E, and condenses the air within; the reaction of the air thus condensed forces water up the tube FG.

During this process the pressure of the water in the large tube diminishes, and the valves C and D both fall; the fall of the latter produces a rush of water through the

Invented by Montgolfier.

opening D, followed by an increased flow down AB, the result of which is again the closing of D, and a repetition of the process just described, the water ascending higher in FG, and finally flowing through G.

The action of the machine is assisted by the air-vessel H in two ways, first, by the reaction of the air in H which is compressed by the descending water, and secondly by the valve K which affords supplies of fresh air. When the water rises through C, the air in H suddenly expands, and its pressure becoming less than that of the outer air, the valve K opens, and a supply flows in, which compensates for the loss of the air absorbed by the water and taken up the column FG, or wasted through D. About a third of the water employed is wasted, but the machine once set in motion will continue in action for a long time provided the supply in the reservoir be maintained.

The Atmospheric Steam Engine.

118. This instrument, constructed by Newcomen soon after the year 1700, was the first in which the oscillation of a beam was maintained by the elastic force of steam.

M

G.

P

A solid beam EGF, which is moveable about G, has its ends arched; to these ends chains are attached which are connected with the rod of a piston in a cylinder AB, and with a rod supporting a weight P, this weight being less than the atmospheric pressure on the piston. C is a

pipe connected with a boiler, B a pipe opening by a stopcock, and D is a pipe connected with a cistern of cold water.

This engine was first used for working the pumps of mines, and a rod Q attached to P is connected with the piston-rod of a pump.

The stop-cocks at C and D are connected with the beam, so that when M is at A, C is closed, and D opens, and when M is at B, C opens and D is closed. The stopcock at B is made to open when M descends to B, and to close immediately after.

Action of the Engine. The pressure of the steam in the boiler is a little greater than that of the atmosphere, and when M is at B, C is open, and steam rushes into MB; hence the weight P will cause the piston to ascend. When M reaches A, C is closed, D is opened, and a jet of cold water is thrown in, condensing the steam, and thereby producing very nearly a vacuum below M. The pressure of the air on the piston being greater than the weight P forces the piston down, and when it has descended, C again opens, and an oscillation of the piston is thus maintained.

As B opens when M descends to the lowest point of its range the water flows out before the ascent.

In the actual engine constructed by Newcomen the stop-cocks were turned by hand, but an attendant, left to work them, invented the machinery by which the engine became self-acting.

The Single-acting Steam Engine.

119. In the atmospheric engine, the cooling of the cylinder at each stroke of the piston causes a great loss of power, for the steam on first entering the cylinder is partially condensed, and its elastic force is therefore diminished. One of Watt's first improvements was to produce the condensation in a separate vessel. The tube D was made to communicate with a vessel containing cold water, the space above the water being a vacuum. This vacuum could be produced by filling the vessel with steam and then

condensing it by cooling the vessel. When the piston is at 4, the stop-cock opens and the steam rushes into the vacuum, and is therefore condensed by the cold water. A pump from the condensing vessel was connected with the beam, so that the overplus of water arising from the condensed steam would be drawn off as soon as formed. These two changes in the atmospheric engine constitute the single-acting engine, but the additional change of making the steam drive the piston downwards as well as upwards, leads to the double-acting engine, the type of most of the steam engines now in actual use.

Watt's Double-acting Steam Engine.

120. The cylinder AB, in which the piston works, is closed at both ends, the piston ranging from a to b. The end of the piston-rod is connected by means of a jointed