118

FOUR-PASSAGED COCK.

boiler and condenser, with the top and the bottom of a cylinder; and that these communications are effected by means of two passages, instead of four. In fig. 56, a circular metallic plate is represented, traversed by two curved passages, the one communicating with the steam tube S and the top of the cylinder by A, the other with the tube E leading to the condenser and the bottom of the cylinder by B; the handle which turns the cock is seen at H; the dotted curve In fig. 57, shows the direction in which it is to be turned.

the handle has been pushed down in this direction, and the communications are reversed. The same letters denote the same passages as in the preceding figure; it follows, that the steam tube S now communicates with the bottom of the cylinder by B, while the top of the cylinder communicates through A with the condenser by E. On the subject of the four-passaged cock, Tredgold observes, that "the simplicity of its action in some degree compensates for its friction, but there is the disadvantage of part of the steam being lost in the pipes at each stroke. Its form should be nearly cylindrical, otherwise its friction and tendency to wear unequally will be increased. When it is ground to fit truly, the pressure of the steam tends to keep the surfaces in contact, and

OF THE ECCENTRIC.

119 to wear the cavity into an elliptical shape; hence it is soon necessary to grind it to fit again." Instead of the four-passaged cock, two double-passaged cocks may be employed. The construction of this instrument may be readily conceived by referring to the preceding figures: if one of the passages be obliterated, the four-passaged becomes a double-passaged cock.

In engines which are worked on the expansive principle, some alterations must be made in the dimensions of the apertures of the four-passaged cock, in order to admit of the steam being cut off at any portion of the stroke. Accordingly, if, in one of the passages, the aperture leading to the cylinder be made larger than the other; if, in the other passage, the aperture leading to the condenser be made larger than the other; and, if the cock be so worked that the steam shall always pass to the cylinder by the former, and to the condenser by the latter, of these enlarged apertures; the cock may then be worked twice, instead of once only, so as to cut off the steam at the first movement, and leave the passage to the condenser open till the second. The expansive working of steam may, however, be more conveniently effected by means of two double-passaged cocks.

MECHANISM OF THE VALVES.

90. Of the Eccentric.-The mechanism by which the valves were worked in the early conditions of the steam engine, has been noticed, at pp. 30, 44, and 60. In these cases, the valves are opened and closed by the reciprocating motion of some part of the engine, as the plug-frame, which, rising and falling with the alternate motions of the piston, offers an easy mechanism for working the valves in pumping engines which have no rotatory motion. Mr. Scott Russell states, that he has seen this method adopted with advantage even in marine engines; he adds, that it is effective, in so far as it at once opens the passages to their fullest extent,

120

OF THE ECCENTRIC.

and so allows full effect to the entering steam, and full clearance to that escaping. Various kinds of mechanism have been more recently adopted for this purpose; but the apparatus most commonly in use in modern double acting engines, is that of the eccentric; and the action of this machinery depends on the revolving motion of some part of the engine, as the fly-wheel. This construction, and mode of action of the eccentric, may be understood by means of the following figure. A section of the shaft of the fly-wheel

is shown at N; this is made to revolve by means of the crank of the engine. On this shaft or axis is fixed a wheel k, which revolves with it, but its motion is eccentric to itthat is, the centre o of the wheel does not coincide with the centre of the shaft, as is evident from mere inspection of the figure; and, hence, the centre o of the wheel moves round the axis of the shaft. The distance of the centre of the wheel from that of the shaft constitutes the amount of eccentricity, and this amount is equal to one half of the range of motion of the valves which are to be worked by this mechanism. The eccentric is encircled by a ring of metal m, which admits of the wheel revolving within it; to this outer ring is attached the arm i, which works upon a centre, and which, by means of a series of other arms or levers h, is connected with the opening and closing of the valves.

The sum of this action consists in the conversion of

THE CONDENSER GAUGE.

121

the rotatory motion of the fly-wheel shaft, into an alternate vertical motion of the valve-rod 7. The advantage of an eccentric wheel is found in its smooth and unintermitting motion, producing the required changes without the perpetual recurrence of a stroke. In large engines, the pressure of the eccentric upon the shaft, is balanced by a weight.

91. Condenser Gauge. -There are two circumstances which determine the effective motions of the piston: one of these is the direct pressure of the steam from the boiler; the other is the resistance of the steam in the condenser. It is important to be enabled to ascertain the relative condition of these two antagonising agents. The pressure of the steam from the boiler is denoted by the steam gauge, already described (p. 81). But a certain quantity of uncondensed vapour, arising from the hot water in the condenser, resists the action of the piston, previously to its being withdrawn by the air-pump. The force of this vapour is ascertained by the condenser gauge, an instrument which is represented in the annexed figure. It consists of a glass tube A B, upwards of thirty inches in length, and open at both ends, the upper end communicating with the condenser C, the lower end being immersed in a cistern of mercury D. The weight of the atmosphere pressing on the surface of the mercury in the cistern, forces this liquid up the tube; the length of the column thus supported in the tube indicates the difference between the pressure of the atmosphere, and that of the vapour in the condenser. On comparing this column of mercury with that of the common barometer, we are enabled to ascertain the force of the vapour in the condenser, every two inches of difference in the columns being equivalent to a force of nearly one pound on the square inch (see p. 9). Tredgold states that the condenser gauge should indicate the state of the vapour in the condenser, to be capable of sustaining from two to three inches of mercury; that,

Fig. 59.

while it does not exceed three inches, the condensation may be esteemed very good, and that about two inches is the best he has seen obtained in practice. The force by which the motion of the piston is determined, is, therefore, ascertained by reference to the steam gauge, and the condenser gauge : the difference between the elastic force of the steam in the boiler, and that in the condenser, added to the height of the barometer at the time, will indicate the relative force of the steam to work the engine. From the resemblance of the condenser gauge to the common barometer, it is frequently called the barometer gauge.

G

92. The Indicator.-But the force of the steam in the cylinder, and the state of exhaustion in the condenser, vary at different portions of the stroke of the engine, and these variations cannot be ascertained by means of the two gauges already described; the mercurial column would be affected by constant vibrations, corresponding with these variations, and it would be impossible to ascertain its mean height during the stroke. To obviate these difficulties, Watt employed an instrument called an indicator. It consists of a cylinder C of about an inch and three quarters in diameter, and eight inches in M length, of very uniform calibre; it terminates below in a pipe, to which a small cock D is adjusted. A solid piston is accurately fitted to the cylinder, so as to move steam-tight within, by the means of oil; the piston-rod G is about five-eighths of an inch in diameter, and sixteen inches in length; to prevent any jar or friction, the rod is made to pass through a guide, H, at a distance of about six inches from the top of the cylinder; the upper part of the piston-rod forms an index, M, which moves upon a graduated scale, K. A spiral spring, I, is attached to the piston, and to the guide H; it is about seven inches in length when at rest, and admits of being compressed an inch and a half;

DI

Fig. 60.