Rising Valves.

436.-The common clack valve is one of the most simple; in its common form it is a plate of leather a little larger than the valve aperture with a part of it fixed in the joint as a hinge. The leather is strengthened by a metal plate on each side, the lower one less, and the upper one larger than the valve aperture. It must open to an angle of about 30°, to allow a free passage equal to its aperture, and the box should be one and a half times the diameter of the valve aperture.

Its chief application in the steam engine is for the valve between the condenser and air pump, called the foot valve, and for the blow valve, but on account of the heat of the water it is necessary to use metal instead of leather, and to grind the parts to fit.

The foot valve G, Fig. 1, and Fig. 4, Plate V. is sometimes suspended by a hinge joint to the upper side of the passage, and falls against an inclined seat, the inclination being as much as to cause the weight of the valve to close it and not more.

437.-A double clack valve, consists of two semicircular valves, and is used for pump buckets; the construction of this is similar to the single clack valve, and the valves must rise to the same angle. They have the advantage of being more convenient than single ones for large pistons. The air pump bucket in a steam engine is furnished with metal valves of this species. See Plate V. p, Fig. 1, and 4.

To afford a greater quantity of passage with less resistance to open the valves, a kind of pyramidical valve consisting of four triangular pieces is sometimes used, but the construction is complex, and without corresponding advantages.

438. A flat metal plate has frequently been recommended for a valve, particularly for a safety valve; it requires a guide sufficient to keep it in its proper seat; which may be most effectively done by a spindle sliding in holes in cross bars, above and below the valve. The diameter of the box should be to that of the valve as 3:2; and the parts should be ground together with emery, till they fit steam-tight. Its advantage as a safety valve is supposed to consist in its being less liable to stick fast, and with this opinion I perfectly agree; in other respects it differs little from the conical valve.

439. The conical steam valve is a plate of metal, with its edge bevelled to fit into a conical seat, it is sometimes called a puppet or T-valve. The steam valves of Watt's engines were at first made of this kind. In this valve the diameter of the box should be to the greater diameter of the valve as 3: 2, and it should rise not less than one-fourth of its greatest diameter when quite open; but both these proportions must be increased if the valve be out of the centre of the box. These valves and seats are often made of brass, gun metal is better, the plate and the seat for it being of the same metal. They are

but

turned as nearly to fit as possible, and afterwards the one is ground into the other, till it accurately fits the seat, with fine emery powder.

The best angle for the valve to fit in its seat is forty-five degrees, for then the pressure is balanced by the reaction of the sides. With less taper the valve has a tendency to set fast, with greater it occupies more space. When the conical valve exceeds five or six inches in diameter, it requires great power to open it against the pressure of the steam, and therefore is inconvenient. Mr. Watt applied a piston to the stem of the valve, fitted to a cylinder of the same diameter as the valve, on the opposite side of the passage, and the steam acting on the valve and piston equally, the difficulty of raising it was much reduced.

When the valve is to be self acting, that is, to move as soon as its narrower surface is exposed to a given pressure, then the weight of the valve must be equal to the square of the diameter multiplied by the pressure in pounds in a circular inch.

440.-A valve is sometimes made with the seat a portion of a sphere, and the valve either a portion or a complete sphere to fit it. This species, under the name of a cup valve, has been strongly recommended for safety valves, and by suspending the weight below the valve it is expected it will in a steam vessel be constantly in motion, so as to prevent sticking. See U, Plate XVII. Fig. 1. In other respects the cup valve seems to be inferior to the conical valve.

441-Hornblower's valve. A common valve must often have to be opened against a pressure depending on its surface; to avoid this, a valve on a different principle was invented by Hornblower. This valve, Fig. 4, Plate VI. is inclosed in a box, and consists of a short cylinder resting on two conical seats, one on the exterior of the cylinder, the other is an interior seat at the bottom of it. The valve is raised or depressed by the usual methods applied to the cross bar at the top, and it is guided by the rod which slides in a socket in the lower seat. If there be strong steam on the upper side of the valve, and light vapour below, the pressure tending to keep the valve close is exerted only on the horizontal areas of the two seats, instead of being distributed over the whole surface of the valve.*

This reduction of pressing surface is obviously considerable in large valves. The principal passage for the steam is very direct, and at the lower seat the steam in its passage going chiefly down through the body of the valve, it is interrupted only by the cross bar at the top.

442.-Improved form for Hornblower's valve. The obvious difficulty of the valve,

* Professor Robison saw the theoretical advantages of this construction, but why has the account he gave of it been omitted in the reprint of his works?

is to make it fit steam-tight on two seats, but if we make the outside of the cylinder to slide in a stuffing box, or in an elastic packing of metal, (see V, Fig.1, Plate VI.) that difficulty is removed, and the largest valves may be made with no other resistance to being opened, than the pressure on the seat, and the friction of the surface of the cylinder. It is simply the common conical valve inverted, and that which formed the seat in the common valve moves instead of the plate; and should obviously slide in a steam-tight case.

Sliding Valves.

443.-The sluice is the oldest form of this valve, but its advantages for any other than rough work in wood do not appear to have been understood: indeed it was not to be expected that metallic surfaces would slide on each other so closely as to be tight and durable, unless very truly worked, and of a hard metal.

Mr. Watt endeavoured to employ them at first but did not succeed, and it was not till more accurate methods of workmanship were introduced about thirty years ago, that the slide valve appeared.

444.-Bramah's slide valve. This slide valve is extensively used for pipes of water works, breweries, gas works, and various other purposes, and is exceedingly well adapted for steam passages. It consists of a box with a slider at right angles to the passage, moved by a rod passing through a stuffing box.

The slider is ground to fit accurately against the circumference of the passage with one surface, and is held close by a spring; it is moved by a handle for small apertures, and for larger ones by a rack and pinion.

445.-The first idea of employing slides for more than one aperture appears to have been to the air pump by Lavoisier or some of his associates, on which Dr. Robison has remarked, that a sliding plate performs the office of four cocks in a very beautiful and simple manner; he adds, however, "that the best workmen in London thought they would be difficult to execute." The same principle was applied to the steam engine by Murray in 1799, a sliding box answering the purpose of opening and closing four steam passages, to use Dr. Robison's words "in a beautiful and simple manner.'

446.-Murray's slide. The apertures all terminate in a steam-tight case, and within this a smaller box slides up and down, so as alternately to open and close the passages. A section of it is shewn in the annexed Plate VI. Fig. 5. The sliding part is moved by the rod o passing through a stuffing box. The steam from the boiler enters at S, and passes through a to the top of the cylinder, when the slide is down, while the passage c

* Art. Pneumatics. Robison's Mech. Phil.

F F

to the condenser is open through the interior portion of the slide; in like manner when the slider is up, the passage b for the steam to the bottom of the cylinder is open, and the passage a from the top to e the condenser is open.

A small reciprocating motion is obviously sufficient for the motion of the slide: its friction from the pressure of the steam against the box is considerable; but in order to reduce it, the rubbing surfaces should not be too sinall, and the harder they are the better; for steam boats gun metal is used, but where salt water is not to be employed, the sliding parts which apply together may be made of steel, and hardened; they then act and wear extremely well.

447.-Murdoch's slides. In slides formed in the preceding manner there is a loss of steam, in consequence of the apertures being opened and closed at some distance from where the steam enters the cylinder. This has been avoided in Messrs. Boulton and Watt's engines, where they have used similar slides invented by Murdoch, in which the strong steam is in the place assigned by Murray to the weak: and in engines with a long stroke, they make the two sliders separate, and move them by a rod of communication; because it would be more difficult to fit a long slide so that there would be a certainty of its rubbing surfaces being in complete contact, as the least deviation of these sliders, whether at the top or bottom of the cylinder, would cause a great leakage. Maudslay also, in his later boat engines, has adopted the same arrangement of slides as Boulton and Watt. See Fig. 2, Plate IV.

448.-Slides are getting into considerable repute for many purposes, and even in appearance the intricacy of a double engine is much diminished by using them. The contrivance of the slide to shut off the steam at any portion of the stroke is a point of some importance. Mr. Millington justly esteems the want of the power to do so a defect, and says it is common to the slide and four-passaged cocks ;* but this objection may be removed in both cases by increasing the quantity of motion of the sliding surfaces one-half. For this purpose the slide should be the depth of the aperture shorter than will cover both the apertures to the cylinder, (see Fig. 1, 2, and 3, Plate V.) and it should be moved twice during the stroke by an adjustable tappet: the first motion shuts off the steam, as in Fig. 2; the second opens the passage to the condenser, and admits the steam at the other end. In this case let F and D represent the passages to the cylinder, S the place where the steam enters, and E the passage to the condenser. Suppose the steam to have been admitted to the upper part of the cylinder by the passage F, Fig. 1, and the slide to have been moved its first motion in Fig. 2, so as to cover F, and still leave D open to the condenser; then, at the next movement, Fig. 3, the slide will be at the bottom and admit

Epitome of Natural Philosophy, p. 313.

1

steam at D, and F will be open to the condenser. The steam should encircle the pipe E; it then does not increase the friction materially by its pressure.

449.-The chief object of attention in setting out a slide, is to shorten its motion as much as possible, so as not to reduce the area of the passages. The area of the rubbing surface can scarcely be estimated at less than eight times that of the passages, which will be about one twenty-fifth of the area of the cylinder, (art. 154,) hence, eight twentyfifths the pressure; and taking the maximum pressure to be double the mean pressure, and the friction being supposed one-eighth of the pressure, it will be two twenty-fifths of the moving force, and it will be, in a short cylinder in action, about one-fifth of the length of the stroke; whence the loss amounts to about one sixty-second of the power of the engine. In long cylinders the ratio will be less.

450.-The cylindrical slide of metal, like a piston in a tube, was applied by Edelcrantz to the safety valve, but such a slide would obviously either be subject to stick fast, or allow steam to escape, as it would bear neither wear nor corrosion. Woolf's slide for regulating the quantity of steam passing an aperture is of the same kind, and seems to have no useful application whatever. The attempt has been made to apply the metallic piston as a slide, and there is no doubt that it may be used both for that purpose and for the back of a flat slide; the object must be to construct it so as to be tight, and wear equally when applied in a cylinder. The advantages of such a slide I have endeavoured to shew in Plates IV. and VI.+

Rotary Valves.

451.-Axis valves are the most simple of the valves moved by rotary motion. A valve of this kind consists of a plate of metal fixed on an axis in the passage; the axis crosses the centre of the plate, and is made to pass through an air-tight aperture to the outside. They are extremely useful where perfect tightness is not required, as in the throttle valve, for dampers and the like. Belidor applied an axis valve to pump work, by putting the axis a

* Philosophical Magazine, Vol. XVII. P. 164.

+ In Fig. 4, Plate IV. I have shewn a mode of construction for the piston slide, which would possess some advantages. A ring, cylindrical on the outside and conical in the inside, may be cut into two or more parts, with lap joints, and these parts may be expanded by the pressure of the steam on a conical part made to fit the interior of the ring; on the opposite side there should be a plate ground to fit the surface of the ring, and between this plate and the bottom of the cone, an elastic packing of hemp should be inserted; and the whole held together by nuts upon the piston rod. The steam apertures should be divided so that no single aperture should exceed one-eighth of the circumference.