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long passage k l into the front part of the large cylinder, and the steam in the lower part of the same is passing from the port 5, through the recess i, into the exhaust-port f. If the slide be now moved forward the steam will then pass from the steam-port e through the recess j, into the port d; from the port c, through i,

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into the port b, and from the port a into the extremity 7 of the long passage and out at the exhaust f. It cannot pass out at the end k of the passage, as that will be closed by contact with the part m of the port face. n is the slide rod; g h is a front view of the slide valve. There are many varieties of slides for doublecylinder engines, some of which exhibit great ingenuity, but the foregoing example may be regarded as a type of most of them, wherefore we shall now take leave of this subject.

The next part of the cylinder apparatus which strikes the observer's eye, is, when such is used, the expansive valve; the object of which is to supply a means of cutting off the steam from the cylinder at any part of the stroke of the piston, independently of the action of the slide valve. These valves are usually of simple construction, and Fig. 65 exhibits a section of one form of expansion valve. Let a b be the slide-valve jacket or steam chest within which the slide valve performs its usual duty. The steam is not admitted direct to this chest, but to another one, cd, placed at the back of it. ef is a flat plate of metal, accurately surfaced and fitted to the back of the slide jacket, in which three, or more

or less, as the case may be, longitudinal slits are made, an equal number of slits being made in the plate ef, and so disposed that

in one position they coincide with the ports in the back of the slide jacket. It is evident that while the expansion valve, as it is called (ef), is in the one position mentioned above, steam will be freely admitted to the cylinder; but a slight movement will cut it off, and the narrower the slits the more suddenly may the steam communication be closed. This motion is imparted to the valve sometimes by an eccentric, but more generally by a cam, as will hereafter be explained. When the slits are very numerous the valve is called a gridiron valve.

While on the subject of sliding valves, we may mention that all the flat surfaces must be

truly planed and scraped, after which operation

Fig. 65.

a

Fig. 66.

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they will present a mottled or patchy appearance, being a very near approach to a true plane, but consisting in reality of small irregular hollows and ridges. When the surface is planed only, it consists of numerous grooves and furrows, but approaches most nearly to an accurate plane when it is truly ground with very fine grounding powder, the operation, however, being so inconvenient that it is not practised for flat work. After the slides have been in use for some time, they present on their surfaces a bright appearance, exercising also a chromatic effect probably due to thin layers of oxide on the surface of the metal polished by friction. In fitting up the valves great care must be taken that when finally set in position they are perfectly free from any kind of grit or dirt, otherwise the faces will be injured.

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b

It sometimes occurs that water collects in the cylinder, and as it cannot be readily expelled through the ports, it is necessary to provide some special means of exit. For this purpose a valve of the form shown (Fig. 66) is frequently used. It consists of a plain

conical valve, with a spindle working in guides or with a stalk. It is kept in its seat by the action of a spring. The apparatus may be screwed into the cylinder cover, or otherwise be made to communicate with it. If any water accumulate in the cylinder, the piston will, at the termination of its stroke, force it out through the valve. a represents a section of a valve and seat, suited to be screwed on to a cylinder cover. The valve is furnished with a spindle moving in guides. b and c exhibit an elevation and bottom plan of a stalk valve removed from its seating.

There is yet another kind of fitting which demands our attention in connection with the cylinder; it is a grease-cock, the object being to supply lubricating material to the internal parts of the apparatus while it is at work. We show at Fig. 67 sections of two forms of grease-cock, both intended to be screwed into the cylinder or other part where they may be required. In the sec

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tion a the lubricating material is poured into the cup b, then by turning the stop-cock between b and c, the grease flows into the reservoir c. This cock is then closed, and the lower one opened, when the grease will flow into the vessel to be lubricated. At e is shown a class of grease-cock suitable for low pressure or condensing engines. The cup is closed at the bottom with a valve, capable of being raised by a screw on turning the handle shown in the section. The method of using it is as follows: when the piston is receding from the grease-cock, and there is consequently a vacuum formed beneath the latter, the valve is opened, upon which

the pressure of the atmosphere forces a portion of the lubricating material into the cylinder.

The cylinder is fitted also with a cock for blowing the steam through both ends at once.

Now that we have described the form and construction of the cylinder, and those parts which are attached to it, we will pass on to the next class of general elements.

CHAPTER XIII.

ON THE DETAILS OF STEAM-ENGINES (Continued).

Pistons, Rods, Beams, Governors, &c., &c.

THE steam-cylinder having been fully discussed, with its external appendages, it is necessary to examine its interior mechanism; the first and most important part of which is the piston, upon which the steam acts directly, and to which it communicates its energy. It is evidently very necessary to exercise much care and ingenuity in order to obtain a satisfactory result; the requirements being that the piston shall fit the cylinder as nearly as possible air and steam-tight, that it shall be as durable as possible, and move with the least possible amount of friction. With the metallic packing now exclusively used, results in practice very satisfactory, fulfilling as nearly as possible the above requirements, have been arrived at, the leakage being equal to an aperture about the fivethousandth of an inch in width.

It is advisable to commence the description with the simplest form of piston, observing, however, previously, that the aperture in the centre of the piston is intended to receive the piston rod.

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Fig. 68, a b, and the following sketches, show views of a simple form of piston. The first view is an elevation, the second a section, and the third a plan. a b are the top and bottom surfaces of the piston, g h a cut packing ring, e da tongue piece, to prevent the leakage of steam where the packing ring is cut through; e ƒ in the section and plan show the body of the piston, i j the junk ring, and the aperture for the extremity of the piston rod. The junk ring has for its office to hold the packing ring in position; and it is attached to the body of the piston by means of bolts. The plan is taken with the junk ring removed, in order to show the packing ring in plan. It will be observed that this packing

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