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mixture may have the temperature of 100°? This will be easily computed. As the cubic inch of water at 1212° is to be reduced to 100°, it must be deprived of 1112° of its temperature. On the other hand, as many inches of water at 60° as are to be added, must be raised in the same mixture to the temperature of 100°, and therefore each of these must receive 40° of temperature. The number of cubic inches of water necessary to be added will therefore be determined by finding how often 40° are contained in 1112°. If 1112 be divided by 40, the quotient will be 27.8. Hence it appears, that to reduce the water in the condenser to the temperature of 100°, supposing the temperature of the water injected to be 60°, it will be necessary to supply by the injection cock very nearly twenty-eight times as much water as passes through the cylinder in the state of steam; and therefore if it be supposed that all the water evaporated in the boiler passes through the cylinder, it follows that about twentyeight times as much water must be thrown into the condenser as is evaporated in the boiler.

In

From these circumstances it will be evident that the cold cistern in which the condenser and air-pump are submerged, must be supplied with a considerable quantity of water. dependently of the quantity drawn from it by the injection valve, as just explained, the water in the cistern itself must be kept down to a temperature of about 60°. The interior of the condenser and air-pump being maintained by the steam condensed in them at a temperature not less than 100°; the outer surfaces of these vessels consequently impart heat to the water in the cold cistern, and have therefore a tendency to raise the temperature of that water. To prevent this, a pump called the cold pump, represented at L in fig. 21., is provided. By this pump water is raised from any convenient reservoir, and driven through proper tubes into the cold cistern. This cold pump is wrought by the engine, the rod being attached to the beam. Water being, bulk for bulk, heavier the lower its temperature, it follows that the water supplied by the cold pump to the cistern will have a tendency to sink to the bottom, pressing upwards the warmer water

water is drained off, and the cistern therefore maintained at the necessary temperature.

From what has been stated, it is also evident that the hot well B, into which the warm water is thrown by the air-pump, will receive considerably more water than is necessary to feed the boiler. A waste-pipe, to carry off this, is also provided ; and the quantity necessary to feed the boiler is pumped up by a small pump, o, the rod of which is attached to the beam, as represented in fig. 21., and which is worked by the engine, The water raised by this pump is conducted to a reservoir from which the boiler is fed, by means which will be hereafter explained.

We shall now explain the manner in which the machine is made to open and close the valves at the proper times. By referring to the explanation already given, it will be perceived that at the moment the piston reaches the top of the cylinder, the upper steam valve G must be open, to admit the steam to press it down; while the exhausting valve I must be opened, to allow the steam to pass to the condenser; and the condensing valve E must be opened, to let in the water necessary for the condensation of the steam; and at the same time the lower steam valve H must be closed, to prevent the passage of the steam which has been admitted through G. The valves G I and E must be kept open, and the valve H kept closed, until the piston arrives at the bottom of the cylinder, when it will be necessary to close all the three valves, G, I, and E, and to open the valve H, and the same effects must be produced each time the piston arrives at the top and bottom of the cylinder. All this is accomplished by a system of levers, which are exhibited in fig. 21. The pivots on which these levers play are represented on the framing of the engine, and the arms of the levers Gʻ, H', and I', communicating with the corresponding valves G, H, and I, are represented opposite a bar attached to the rod of the airpump, called the plug frame. This bar carries certain pegs and detents, which act upon the arms of the several levers in such a manner that, on the arrival of the beam at the extremities of its play upwards and downwards, the levers are so struck that the valves are opened and closed at the proper

times. It is needless to explain all the details of this arrangement. Let it be sufficient, as an example of all, to explain the method of working the upper steam valve G. When the piston reaches the top of the cylinder, a pin strikes the arm of the lever G', and throws it upwards: this, by means of the system of levers, pulls the arm of the valve G downwards, by which the upper steam valve is raised out of its seat, and a passage is opened from the steam pipe to the cylinder. The valve is maintained in this state until the piston reaches the bottom of the cylinder, when the arm Gʻ is pressed downwards, by which the arm & is pressed upwards, and the valve restored to its seat. By similar methods the levers governing the other three valves, H, I, and E, are worked.

A

The valves used in these engines were of the kind called Fig.23. spindle valves. They consisted of a flat circular plate of bell metal, A B, fig. 23., with a round spindle passing perpendicularly through B its centre, and projecting above and below it. This valve, having a conical form, was fitted very exactly, by grinding into a corresponding circular conical seat, A B C D, fig. 24., which forms the passage which it is the office of the and close. When the valve falls into its seat,

valve to

open

it fits the aperture like a plug, so as entirely to stop it. The

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spindle plays in sockets or holes, one above and the other below the aperture which the valve stops; these holes keep the valve in its proper position, so as to cause it to drop exactly into its place.

In the experimental engine made by Mr. Watt at Kin

neal, he used cocks, and sometimes sliding covers, like the regulator described in the old engines; but these he found very soon to become leaky. He was, therefore, obliged to change

ground, and accurately fitted in the first instance, were not so liable to go out of order. These valves are also called puppet clacks, or button valves.

In the earlier engines constructed by Watt, the condensation was produced by the contact of cold surfaces, without injection. The reason of rejecting the method of condensing by injection was, doubtless, to avoid the injurious effects of the air, which would always enter the condenser, in combination with the water of condensation, and vitiate the vacuum. It was soon found, however, that a condenser acting by cold surfaces without injection, being necessarily composed of narrow pipes or passages, was liable to incrustation from bad water, by which the conducting power of the material of the condenser was diminished; so that, while its outer surface was kept cold by the water of the cold cistern, the inner surface might, nevertheless, be so warm that a very imperfect condensation would be produced.

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CORRESPONDENCE OF WATT WITH SMEATON. FAILURE OF CONDENSATION BY SURFACE.

-METHOD OF PACKING.

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EXPEDIENTS

IMPROVEMENTS IN CONSTRUCTION OF PISTON. IMPROVEMENTS IN BORING THE CYLINDERS. DISADVANTAGES OF THE NEW COMPARED WITH THE OLD ENGINES. - GREATLY INCREASED ECONOMY OF FUEL. TO FORCE THE NEW ENGINES INTO USE. CORRESPONDENCE WITH SMEATON. EFFICIENCY OF FUEL IN THE NEW ENGINES. DISCOVERY OF THE EXPANSIVE ACTION OF STEAM.- WATT STATES IT IN A LETTER TO DR. SMALL. ITS PRINCIPLE EXPLAINED. MECHANICAL EFFECT RESULTING FROM IT.- COMPUTED EFFECT OF CUTTING OFF STEAM AT DIFFERENT PORTIONS OF THE STROKE. PRODUCES A VARIABLE POWER.-EXPEDIENTS FOR EQUALISING THE POWER.LIMITATION OF THE EXPANSIVE PRINCIPLE IN WATT'S ENGINES. ITS MORE EXTENSIVE APPLICATION IN THE CORNISH ENGINES.

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(77.) IN a letter addressed by Watt to Smeaton, dated April, 1766, Watt refers to some of these practical difficulties which he had to encounter. "I have been," says he, " tormented with exceedingly bad health, resulting from the operation of an anxious mind, the natural consequence of staking every

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