266 LECTURE XXIX. 



great measure avoided in Newcomen's engine,* where the steam was gra- 

 dually introduced into a cylinder, and suddenly condensed by a jet of 

 water, so that the piston was forced down with great violence by the pres- 

 sure of the atmosphere, which produced the effective stroke : this effect 

 was, however, partly employed in raising a counterpoise, which descended 

 upon the readmission of the steam, and worked a forcing pump in its re- 

 turn, when water was to be raised. The condensation, although rapid, was, 

 however, neither instantaneous, nor complete, for the water injected into 

 the cylinder had its temperature considerably raised by the heat emitted 

 by the steam during its condensation ; it could only reduce the remaining 

 steam to its own temperature, and at this temperature it might still retain 

 a certain degree of elasticity ; thus, at the temperature of 180 steam is 

 found to be capable of sustaining about half the pressure of the atmosphere, 

 so that the depression of the piston must have been considerably retarded 

 by the remaining elasticity of the steam, when the water was much heated. 

 The water of the jet was let off when the piston was lowest, and was after- 

 wards pumped up to serve the boiler, as it had the advantage of being 

 already hot. This engine, with Beighton's apparatus for turning the cocks, 

 was until lately in general use, and it is still very frequently employed. In 

 this, as well as in other steam engines, the boiler is furnished with a safety 

 valve, which is raised when the force of the steam becomes a little greater 

 than that of the atmospheric pressure ; and it is supplied with water by 

 means of another valve, which is opened when the surface of the water within 

 it falls too low, by the depression of a block of stone which is partly supported 

 by the water. (Plate XXIV. Fig. 335, 336.) 



The cylinder of Beighton's machine is necessarily much cooled by the 

 admission of the jet, and by exposure to the air. Mr. Watt has avoided 

 this inconvenience by performing the condensation in a separate vessel, into 

 which a small jet is flowing without intermission ; and by introducing the 

 steam alternately above and below the piston, the external air is wholly ex- 

 cluded ; the piston rod working in a collar of leathers, so that the machine 

 has a double action, somewhat resembling that of Lahire's double pump ; 

 and the stroke being equally effectual in each direction, the same cylinder, 

 by means of an increased quantity of steam, performs twice as much work 

 as in the common engine. We might also employ, if we thought proper, a 

 lower temperature than that at which water usually boils, and work in this 

 manner with a smaller quantity of steam ; but there would be some diffi- 

 culty in completely preventing the insinuation of the common air. On the 

 other hand, we may raise the fire so as to furnish steam at 220 or more, 

 and thus obtain a power somewhat greater than that of the atmospheric pres- 

 sure ; and this is found to be the most advantageous mode of working the 

 engine ; but the excess of the force above the atmospheric pressure cannot 

 be greater than that which is equivalent to the column of water descending 

 to supply the boiler, since the water could not be regularly admitted in 

 opposition to such a pressure. The steam might also be allowed to expand 

 itself within the cylinder for some time after its admission, and in this 

 manner it appears from calculation that much more force might be obtained 

 * His patent is dated 1705.. 



