thoroughly practical nature worth drawing the attention of students to occurs, until Captain Thomas Savery brought out his patent steam engine for raising water from mines in 1698.

C

G for Globe.

Caldron, containing water. N1, N2 Nozzles, steam exhaust. Р Pipe, steam supply.

Savery's Engine.-Steam from the boiler, B, is admitted to the receiver, R, by opening the steam cock, S C. When the receiver is filled with steam, the cock, SC, is closed, and CJ opened, which allows a douche of cold water to play on the outside of R, thus causing condensation and producing a vacuum. The atmospheric pressure acting on the mine water, at M W, forces water up through the suction valve (or cock), S V, nearly filling the receiver. CJ is then closed, and SC opened, thus permitting

the steam from the boiler to force the water now in the receiver up through the delivery valve (or cock), DV, and the discharge pipe, DP, to any convenient place clear of the mine.

In Savery's actual engine he adopted a complete duplex set of

boilers, receivers, and cocks, so that the operations of filling one receiver and emptying the other might be conducted simultaneously. He placed his boilers and receivers about 20 feet above

*

the bottom of the mine water, or well, and the height of the overflow from the discharge pipe about 30 feet above the receiver. The efficiency of a Savery engine, as measured by the weight of coal consumed, was tested by Smeaton, in 1774, and found to be about of what can now be realised by a modern pumpingengine. The loss of heat-energy, due to the alternate heating and cooling of the receiver, added to the condensation of the steam upon coming into direct contact with the water when forcing the latter out of the receiver, combined with the impossi

* Desagulier, in 1716, improved upon Savery's engine by introducing a two-way cock between the boiler, the receiver, and the cold water injection, and introduced an inside rose injection for condensing the steam in the receiver. See Stuart on The Steam Engine, 1825, Fig. 20. 2

bility of placing the receiver much more than 20 feet above the bottom of the mine,* and the inability of engineers in those days to construct boilers of sufficient strength to withstand a steam pressure more than 15 lbs. on the square inch,† prevented the adoption of Savery's engine in most mines.

Newcomen's Atmospheric Engine.--In 1705 Thomas Newcomen, a blacksmith, associated with Savery and John Cawley, a glazier, made the experiment of introducing steam under a piston moving in a cylinder. They formed a vacuum by condensing the steam by an affusion of cold water on the outside of the steam vessel; and the weight of the atmosphere pressed the piston to the bottom of the cylinder. This was the first form of atmospheric engine the simplest and most powerful machine that had hitherto been constructed. After a great many laborious attempts at Wolverhampton to make one of their engines work satisfactorily, they were one day (in March, 1712) surprised "to see the engine go several strokes, and very quick together, when, after a search, they found a hole in the piston, which let the cold water in to condense the steam in the inside of the cylinder, whereas before they had always done it on the outside." This fortunate observation gave rise to the improvement of condensing by injection, which thus rendered the cold water jacket of their steam cylinder unnecessary, and they thereafter manufactured their engines in the form shown in the following figure.

The mine pumps, MP, weighted pump-rod, WP R, and lift pump, LP, on the one side of the wooden beam, W B, being heavier than the piston, P, and piston rod, PR, always brought the piston to the top of the cylinder, C; consequently, to start the engine, the steam valve, S V, was opened, in order to expel the air by the relief or snifting valve, RV,‡ and to fill the whole cylinder with steam. The steam valve was now closed, and the injection cock, I C, opened, which caused a spray of cold water from the cold water tank, CW T, to enter the cylinder and condense the steam. The vacuum produced brought the pressure of the atmosphere into play on the top side of the piston, causing it to descend to the bottom of the cylinder, thus actuating the pumps at the other end of the beam. The condensed steam and injection water got clear away from the bottom of the

*With even a perfect vacuum in the receiver, the atmospheric pressure, which is usually about 15 lbs. on the square inch, could only force water up into it from a depth of 34 feet.

+ Savery said, "If I could only get boilers and pipes of sufficient strength, I could force water up to a height of 1,000 feet."

This valve was called the snifting valve by Newcomen, because the air makes a noise every time it blows through it.

cylinder by the eduction pipe, EP, to the feed water tank, FWT; the water from this tank being used to fill the boiler,

and the height of the water in the boiler was ascertained by

the gauge pipes, G P.

At first (in 1712) the valves were opened and shut by hand. To perform these operations at the precise moment, required the most exact and unremitting attention on the part of the attendant, as the least neglect or inadvertence might be ruinous to the engine, by beating out the bottom of the cylinder, or allowing the piston to be drawn out of it. Stops were contrived to prevent both of these accidents; then strings were used to connect the handles of the cocks and valves with the beam, and finally a Mr. Beighton, in 1718, simplified the whole of these movements by causing them to be automatically opened and shut at the proper moment by means of a "tappet rod" connected with the beam. He also introduced the lever safety valve to the boiler.

Another difficulty which at first severely taxed the ingenuity of the inventors was the sudden upheaving of the cylinder, at the moment of creating the vacuum, which caused such a jolt and stress on the pipes connecting the cylinder and the boiler, as to keep them in a chronic state of leakage. It will be observed that at the instant the vacuum is produced, the piston is pressed downwards by the atmospheric pressure, but at the same time the cylinder is equally pressed upwards, so that it required to be very heavy or very securely fastened down, to prevent it rising; since no downward motion of the piston can take place until the inertia of the whole moving mass of beam, pump-rods, &c., has been overcome. This difficulty was in a measure mastered by bolting the cylinder firmly down to strong beams, and keeping it separate from the boiler.

Newcomen's piston, which consisted of a flat plate with a broad piece of leather screwed to it and turned up the sides of the cylinder two or three inches, gave considerable trouble,