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sphere; in this is placed a glass tube, open at the lower end and closed at its upper. A small iron tube is provided with a stop-cock, and connected with the condenser; this tube passes through the mercury in the cup, and up the interior of the tube near to its top. The air is exhausted from the

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fig. 110.

fig. 109.

glass tube through the iron tube connected with the condenser, as the mercury rises in the glass tube in proportion to the difference between the pressure of the uncondensed vapour in the condenser and the pressure of the atmosphere. To show the higher vacuums of 29 and 30, the height of this gauge must be nearly three feet. To obviate the inconvenience attending this form of gauge, the short vacuum gauge is used, as in fig. 110. A small glass tube contains the mercury, and is filled carefully as an ordinary barometer; it is bent upwards at the bottom, and ends in a bulb, which is provided with a small orifice at the upper side. The tube is attached to a scale, and entirely enclosed in a glass case, which is carefully cemented to a brass cup, terminating in a stop-cock and pipe connected with the condenser. The air in the interior of the glass case is always of the same density as in the condenser. In the long vacuum gauge, in fig. 109, the mercury is driven frequently out of the cup, if the stop-cock is left open while blowing through previous to starting. The short vacuum gauge, although possessed of many advantages, as shortness and compactness, has also disadvantages; these are, first, the vapour from the condenser deposits frequently a mist in the glass case so dense as to obscure the scale; and

secondly, if the stop-cock is not shut while blowing through, the case becomes filled with steam or with hot water; and its safety is thereby endangered. To obviate these inconveniences, Mr. Bramwell, of London, has

designed an improved vacuum gauge, which he described before the Institution of Mechanical Engineers, and of which we give the diagram in fig. 111. "Instead of immersing the whole of the tube and scale in a glass chamber connected with the condenser, the bulb only is enclosed in a brass cup with a screw lid, on which the scale is cast; and the rest of the mercurial tube is passed through a st ng-box in the middle of this lid, protecting it from injury by pushing it in a depression in the scale like a common thermometer. On the bottom of the brass cup is a stop-cock, with the pipe by which connection is made with the condenser; the same density is always preserved in it and the cup; and thus, the pressure being removed from the surface of the mercury in the bulb, it of course falls according to the rarefaction, a fall that can always be observed, as the tube containing the mercury is totally uncovered. By this means, the first and great objection to the short vacuum gauge is done away; and likewise the second, which is common to both long and short, viz. the risk of the stop-cock being left open while blowing through; as with this gauge it is a matter of perfect indifference whether it is open or not, as the only thing that can take place if it is open is, that the brass cup is filled with steam; but this can neither blow out the mercury nor damage the gauge."

fig. 111.

In the companion volume to this treatise, Mechanics and Mechanism, we have described pretty fully the details of construction of the various parts of steam-engines, as connecting-rods, cranks, &c. and the method of putting the various parts together; we are, therefore, spared the necessity of here giving them, and proceed to the consideration of other matters worthy of notice.

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Of contrivances for regulating the speed of steam-engines, the centrifugal-governor is the best known: the arrangements and method of attachment of this mechanism we have already, in Mechanics and Mechanism, fully described. In the pumping-engine the ordinary governor is not used, but a contrivance known as the "cataract" is adopted instead. This was in use prior to the period of the introduction of Watt's engines, and has since been much improved. The principle of the original cataract will be easily understood by an inspection of the diagram in fig. 112. Suppose a to be a vessel for containing water; on the end of the lever m, jointed at c, another lever, f, is attached to and forms part of the lever m; water is allowed to pass into the vessel a through the pipe b; as soon as the a fills, it tilts over in the direction of the arrow, and lifts up the end of lever f, as shown by the dotted lines: to the end off a chain e is attached; this is connected with the end of the lever opening the injection-valve. The vessel a can only tilt over in the one direction, the stud d preventing it from falling the wrong way;

fig. 112.

when empty, the vessel a is brought up to its original position by the weight of the lever f, &c. The whole is contained in a box, from which the water is led away after working the cataract. The number of times the injection-valve is opened determines the number of strokes the engine makes in a given time; and the falling of the box or vessel a, which opens the injection-valve, is regulated by the quantity of water allowed to pass through the pipe b; this quantity of water is therefore proportioned to the quantity to be drawn from the mine, or the work to be done by the engine. The reader will find descriptions of forms of improved cataracts in the Artisan Treatise on the Steam-Engine.

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In cotton-mills, and in cases where the engine is loaded to a certain extent, accidents frequently occur by parts of the machinery breaking down. The engine, thus relieved of a certain part of its load, "runs away," as it is termed, at a greatly increased speed, endangering the stability of the whole apparatus. The ordinary mechanism of the throttle-valve and governor is not found capable of making the engine recover its usual speed. ingenious contrivance, highly esteemed, is given in the following diagram, fig. 113. The main object of this invention is to prevent damage by the accidental alteration of load, and to regulate the speed of steam-engines. It is intended to supersede the common throttle-valve, and prevent variation of speed and the many breakages arising from accidental alterations of weight. The valves can readily be connected with ordinary governors; and when attached, are so sensitive, that should a shaft break, or any weight be suddenly thrown on or off, the engine will recover its usual speed in less than two revolutions, without any interference by the engineer. This will be obvious, when it is considered that there are two valves fitted to the same spindle; and consequently, that each of them need be opened to only one half the extent, as if there were one valve. The result is, a throttle-valve of extreme sensibility, half an inch being the utmost amount of play allowed to the spindle between wide open and quite shut; therefore, a very slight change in the position of the governor-balls produces considerable change in the amount of opening afforded by the valves.

The governor is connected with the valve-spindle by a fork, in the usual manner, which, by means of a crank-lever, gives a slight motion on its axis to a bar, provided at the other end with another crank-lever connected with the valve-spindle. It may be attached to all ordinary governors. In the case of two fifty-horse engines coupled together in Sir C. Armitage's mill, at Pendleton, near Manchester, the result of a trial by throwing off the whole weight was the recovery of the usual speed in 1 revolutions. The load on these two engines consisted of 20,000 throstlespindles, 13,000 mule-spindles, and 250 power-looms, with all the necessary apparatus for working the mill.

In fig. 113 we give a diagram illustrative of this apparatus, so capable of instantaneously regulating the performance of engines. a is the steam-pipe from boiler, e c the double-beat valve; when this is raised by the lever actuated on by the lever d of the connecting-rod, the steam passes from ɑ in the direction of the arrows to the steam delivery-pipe b.

The "hydraulic motion regulator" is an American invention, recently introduced into this country, and which has already taken a high place as an effective governor for steam-engines; we give a diagram and description of this apparatus, taken from the inventor's circular (Mr. Pitcher).

The principle of the invention consists in working a small pump, the water delivered by which acts in the plunger of a second cylinder or barrel: the

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fig. 113.

water from the pump escapes through an aperture of a certain area; this is calculated so as to maintain the plunger at a certain height in the cylinder. Should, however, the speed of the engine increase beyond its regular working speed, the pump is in consequence worked faster, and a greater quantity of water delivered to the plunger cylinder: but the aperture for its escape remaining unchanged, it cannot get away, it therefore fills the cylinder to a higher level; this raises the plunger, which, connected with the regulating-valve or throttle-valve, lessens the supply of steam to the cylinder and reduces the speed. The pump is worked by the engine by any of the ordinary methods of connection. In fig. 114, e is the pump worked by the engine; c the suction-valve, through which the supply of water is obtained, the delivery-valve, supplying the cylinder f. To prevent the plunger from rising higher than necessary, a small hole, g, is made in the cylinder; when the plunger rises above this the water escapes by it, and the plunger rises no higher. The piston-rods pass through the bonnet or cover of the external casing jj; cups ik are used to return any water that may be drawn up through the stuffing-boxes; the piston-rods are thus lubricated, or made to work smoothly, with water surrounding them. The whole apparatus is enclosed in the casing jj; so that the same water is used over and over again. A spring is coiled round the piston-rod of plunger ƒ at z; this prevents its falling further than necessary. The opening by which the water ordinarily escapes is made just above the delivery-valve b; and the amount of opening is regulated by a handle or lever which passes through the cover h, according to the speed at which the engine is desired to run. In place of the ordinary throttle-valve, the inventor prefers to use a regulating valve similar in principle to the disc-valve employed as the regulator” in locomotive engines.

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In calculating the power of steam-engines, there are two terms used— the "nominal power" and the "actual, or effective power." By the term "nominal power," reference is made to an engine having a cylinder of

given diameter, a given length of stroke, with a uniform pressure upon the piston of 7 lbs. per inch. Watt calculated the effective pressure on the piston in small engines at 6.8 lbs.

per inch, and in large-as 100 horse-power-at 6.94; 7lbs., however, is the effective pressure calculated. By the term "actual" power, is meant the number of times 33,000lbs. the engine is capable of lifting 1 foot high per minute. By the term "duty" of an engine is meant the amount of work done in relation to the amount of fuel consumed.

In calculating the nominal horse-power of an engine, the following rule is adopted: "Square the diameter of the cylinder, and multiply the number of square inches thus found by the cube-root of the length of the stroke in feet, and divide the product by 47; the quotient is the number of nominal horse-power of the engine."

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By the term "horse-power," as introduced by Watt, was meant the mechanical force necessary to lift 33,000 lbs. 1 foot high per minute. Prior to the extended introduction of steam-engines, the work at mines, &c. was frequently done with horse-power; it became, then, of importance to be able to state the amount of work done by a steam-engine, as compared with a given number of horses. The power of a horse, as calculated by Watt, was equal to the raising of 33,000lbs. 1 foot high in a minute. Engines now, however, calculated at this rate, really exert a greater power than the nominal power; it is, therefore, of importance to be able to calculate the effective or actual power of an engine, without reference to its nominal power. This is ascertained by means of the "indicator," which gives the effective pressure on the cylinder of the engine: from this is deducted a pound and a half of pressure absorbed in friction, &c.; the velocity of the motion of the piston in feet per minute is thus ascertained: this is done by multiplying the number of revolutions of the engine per minute by the length of stroke. These data having been ascertained, the

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fig. 114.

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