adjusting the position of the cock, so that a part of the opening in it shall be covered by the side of the tube.

(138.) It is sometimes required to put one tube or passage alternately in communication with two others. This is accomplished by a two-way cock. In this cock the passage is curved, opening usually at points on the surface of the cock, at right angles to each other. Such a cock has already been described, and its use illustrated in the description of the Marquis of Worcester's engine (17.); the two-way cock, as represented at K and R (fig. 4.), being the means by which steam and water are alternately supplied to the two forcing vessels.

(139.) When it is required to put four passages alternately in communication by pairs, a four-way cock is used. Such a Fig. 59. cock has two curved passages (fig. 59.), each similar to the curved passage in the two-way cock. Let S CBT be the four tubes which it is required to throw alternately into communication by pairs. When the cock is in the position fig. 59., the tube s communicates with T, and the tube c with B. By turning the cock through a quarter of a revolution, as in fig.60., the tube s is made to communicate with B, and the tube c with T ; and if the cock continue to be turned at intervals through a quarter of a revolution, these changes of communication will continue to be alternately made. It is evident that this may be accomplished by turning the cock continually in the same direction.

The four-way cock is sometimes used as a substitute for the valves or slides in a double-acting steam engine to conduct the steam to and from the cylinder. If s represent a pipe conducting steam from the boiler, c that which leads to the condenser, T the tube which leads to the top of the cylinder, and B that which leads to the bottom, then when the cock is in the position (fig. 59.), steam would flow from the boiler to

the top of the piston, while the steam below it would be drawn off to the condenser; and in the position (fig. 60.), steam would flow from the boiler to the bottom of the piston, while the steam above it would be drawn off to the condenser. Thus by turning the cock through a quarter of a revolution towards the termination of each stroke, the operation of the machine would be continued.

One of the disadvantages which is inseparable from the use of a four-way cock for this purpose is the loss of the steam at each stroke, which fills the tubes between the cock and the ends of the cylinder. This disadvantage could only be avoided by the substitution of two two-way cocks (138.) instead of a four-way cock. A two-way cock at the top of the cylinder would open an alternate communication between the cylinder and steam pipe, and the cylinder and condenser, while a similar office would be performed by another two-way cock at the other end.

The friction on cocks of this description is more than on other valves; but this is in some degree compensated by the great simplicity of the instrument. When the cock is truly ground into its seat, being slightly conical in its form, the pressure of the steam has a tendency to keep the surfaces in contact; but this pressure also increases the friction, and has a tendency to wear the seat of the cock into an elliptical shape. Consequently, such cocks require to be occasionally ground and refitted.

(140.) The four-way cock, as above described, admits the steam to one end of the piston at the same moment that it stops it at the other end. It would therefore be inapplicable where steam is worked expansively. A slight modification, however, analogous to that already described in the slides, will adapt it to expansive action. This will be accomplished by giving to one of the passages through the cock one aperture larger than the other, and working the cock so that this passage shall always be used to conduct steam to the cylinder; also by enlarging both apertures of the other passage, and using it always to conduct steam from the cylinder. The effect of such an arrangement will be readily understood.

Let the position of the cock at the commencement of the

Fig. 61.

descending stroke be represented in fig. 61. Steam flows from s through T to the top of the cylinder, while it escapes from в through c from the bottom of the cylinder. When the piston has arrived at that point at which the steam is to be cut off, let the cock be shifted to the position represented in fig. 62. The passage of steam from the boiler is now stopped, but the escape

of steam from the bottom of the cylinder through c continues,

Fig. 62.

S

and the cock is maintained in this position until the piston approaches the bottom of the cylinder, when it is further shifted to the position represented in fig. 63. Steam now flows from s through B to the bottom of the cylinder, while the steam from the top of the cylinder escapes through c to the conWhen the piston has arrived at that point where the steam is to be cut off, the cock is shifted to the position represented in fig. 64. The communication' between the

denser.

Fig. 63.

Fig. 64.

S

S

steam and the bottom of the piston is now stopped, while the communication between the top of the cylinder and the . condenser is still open. During the next double stroke of the piston the position of the cock is similarly changed, but in the contrary direction, and in the same way the motion is continued. Under these circumstances the cock, instead

R

of being moved constantly in the same direction, as in the case of the common four-way cock, will require to be moved alternately in opposite directions.

PISTONS.

(141.) The office of a piston being to divide a cylinder into two compartments by a movable partition which shall obstruct the passage of any fluid from one compartment to the other, it is evident that the two conditions which such an instrument ought to fulfil are, first, that the contact of its sides with the surface of the cylinder shall be so close and tight throughout its entire play that no steam or other fluid can pass between them; secondly, that it shall be so free from friction, notwithstanding this necessary tightness, that it shali not absorb any injurious quantity of the moving power.

Since, however accurately the surfaces of the piston and cylinder may be constructed, there will always be in practice more or less imperfection of form, it is evident that the contact of the surface of the piston with the cylinder throughout the stroke can only be maintained by giving to the circumference of the piston sufficient elasticity to accommodate itself to such inequalities of form. The substance, whatever it may be, used for this purpose, and by which the piston is surrounded, is called packing.

In steam pistons the material used for packing must be such as is capable of resisting the united effects of heat and moisture. Hence leather and other animal substances are inapplicable.

The packing used for steam pistons is therefore of two kinds, vegetable packing, usually hemp, or metallic packing.

The common hemp-packed piston has been already in part described (79.). The bottom of the piston is a circular plate just so much less in diameter than the cylinder as is sufficient to allow its free motion in ascending and descending. A little above its lowest point this plate begins gradually to diminish in thickness until its diameter is reduced to from one to two inches less than that of the cylinder, leaving therefore around

it a hollow space, as represented in fig. 65. The cover of the

low space which thus surrounds the piston a packing of unspun hemp or soft rope, called gasket, is introduced by winding it round the piston so as to render it an even and compact mass. When the space is thus filled up, the top of the piston is attached to the bottom by screws.

The curved form of the space within which the hempen packing is confined is such that when the screws are tightened, that part of the packing which is nearest to the top and bottom of the piston is forced against the cylinder so as to produce upon the two parallel rings as much pressure as is necessary to render it steam-tight. When by use the packing is worn down so as to produce leakage, the cover of the cylinder must be removed, and the screws connecting the top and bottom of the piston tightened: this will force out the packing and render the piston steam-tight. This packing is lubricated by melted tallow let down upon the piston from the funnel inserted in the top of the cylinder, furnished with a stop-cock to prevent the escape of steam. The lower end of the piston-rod is formed slightly conical, the thickest part of the cone being downward. It is passed up through the piston, and a nut or wedge between the top and bottom is inserted so as to secure the piston in its position upon the rod.

The process of removing the top of the cylinder for the purpose of tightening the screws in the piston is one of so laborious a nature, that the men entrusted with the superintendence of these machines are tempted to allow the engine to work notwithstanding injurious leakage at the piston, rather than incur the labour of tightening the screws as often as it is necessary to do so.

To avoid this inconvenience, the following method of