424 Hydrodynamics. 



valve S'. The communication between S and S' is effected by 

 means of a curved pipe, whose plane of curvature is perpendic- 

 ular to the plane of projection in figure 253. One part only of 

 this pipe is represented in this figure ; in order that the two valves 

 S", S'", may be exhibited, of which we are presently to speak ; 

 but the whole may be seen in figure 254, where it is presented in. 

 profile. The valves S", S'", are those by which the steam of the 

 cylinder is brought into communication with the condenser, on 

 both sides of the piston ; and they are opened and closed at proper 

 times by the engine itself, with the aid of two small projections, 

 1, 2, attached to the rod 1 1 of the pump which serves to exhaust 

 the condenser 0. This opening is effected a little before the 

 piston has completed its vertical motion, and the communication 

 is then established between the two surfaces, in order that the 

 equality of pressure thence resulting may weaken the effort 

 which would be made if it were exerted on one side only, and 

 prevent the sudden jar which would follow from the piston's 

 striking with its full force against the bottom of the cylinder. 



These are the principal conditions relating to the action of 

 the steam, but there are others which relate to the manner of 

 directing this action. Indeed, a bare inspection of the figure 

 will show that the rod of the great piston and that of the pump 

 which exhausts the condenser, being both inflexible, cannot be 

 attached immediately to the large lever JIB ; for each point of 

 this lever, describing an arc of a circle about its centre of rota- 

 tion, would tend to change the point of attachment from a ver- 

 tical direction, and this effort would break the engine. It is on 

 this account, that, in the engine of Newcomen, where the piston 

 only acted in its descent, its communication with the large lever 

 was made by a chain applied to the arc of a circle. But in the 

 engine under consideration, the inflexibility of the rods requires 

 some other mode of communication. Mr Watt effected this by 

 means of a particular assemblage of metallic bars, moving upon 

 one another, and which compensate by their action, for the want 

 of perfect verticality in the motion of the large lever. The figure 

 represents also, several other useful appendages to the engine, 

 such as flies to regulate its motion, and wheels to transmit it. 

 There is also a very essential part designated by G, and called 

 the governor. It consists of a vertical rod which is kept contin- 



