and vice versa. If we suppose another lever N O connected with N M, and 

 at right angles to it, forming what is called a bell-crank, then the alternate 

 motion received by M, from right to left, would give a corresponding motion to 

 the extremity O of the lever N O, upward and downward. If this last point 

 O were attached to a vertical arm or shaft, it would impart to such arm or 

 shaft an alternate motion upward and downward, the extent of which would be 



5 regulated by the length of the levers respectively. 



S By such a contrivance the revolution of the fly-wheel shaft is*made to give 

 an alternate vertical motion of any required extent to a vertical shaft placed 

 near the cylinder, which may be so connected with the valves as to open and 

 close them. Since the upward and downward motion of this vertical shaft is 

 governed by the alternate motion of the centre C to the right and to the left 

 of the centre G, it is evident that by the adjustment of the eccentric upon the 

 fly-wheel shaft, the valves may be opened and closed at any required position 

 of the fly-wheel and crank, and therefore at any required position of the piston 

 in the cylinder. 



Such is the contrivance by which the valves, whatever form may be given 

 to them, are now almost universally worked in double-acting steam-engines. 

 Having described the general structure and operation of the steam-engine as 



! improved by Watt, we shall now explain, in a more detailed manner, some 

 parts of its machinery which have been variously constructed, and in which 

 more or less improvements have been made. 



OF THE COCKS AND VALVES. 



In the steam-engine, as well as in every other machine in which fluids act, 

 it is necessary to open or close, occasionally, the tubes or passages through 

 which these fluids move. The instruments by which this is accomplished are 

 called cocks or valves. 



Cocks or valves may be classified by the manner in which they are opened: 

 1st, they may be opened by a motion similar to the lid of a box upon its 

 hinges ; 3d, they may be opened by being raised directly upward, in the same 

 manner as the lid of a pot or kettle ; 3d, they may be opened by a sliding mo- 

 tion, like that of the sash of a window or the lid of a box which slides in 

 grooves ; 4th, they may be opened by a motion of revolution, in the same 

 manner as the cock of a beer-barrel is opened or closed. The term valve is 

 more properly applied to the first and second of these classes ; the third class 

 are usually called slides, and the fourth cocks. 



The single clack valve is the most simple example of the first class. It is 

 usually constructed by attaching to a plate of metal larger than the aperture 

 which the valve is intended to slop, a piece of leather, and to the under side 

 of this leather another piece of metal smaller than the aperture. The leather 

 extending on one side beyond the larger metallic plate, and being flexible, 

 forms the hinge on which the valve plays. Such a valve is usually closed by 

 its own weight, and opened by the pressure of the fluid which passes through 

 it. It is also held closed more firmly by the pressure of the fluid whose re- 

 turn it is intended to obstruct. An example of this valve occurs in the steam- 

 engine, in the passage between the condenser and the air-pump. The aperture 

 which it stops is there a seat inclined at an angle whose inclination is such as 

 to render the weight of the valve sufficient to close it. In cases where the \ 

 valve is exposed to heat, as in the example just mentioned, where it is con- < 

 tinually in contact with the hot water flowing from the condenser to the air- \ 

 pump, the use of leather is inadmissible, and in that case the metallic surface < 

 of the valve is ground smooth to fit its seat. 



The extent to which such a valve should be capable of opening, ought to 



