ON PNEUMATIC MACHINES. ' 347 



* 



heit, and for a column of 68 feet, to i271°; such a pressure, also, acting 

 on the internal surface of the vessels, made it necessary that they should be 

 extremely strong; and the height to which water could be drawn up from 

 below, when the steam was condensed, was limited to 33 or 34 feet. A still 

 greater objection was, however, the great quantity of steam necessarily 

 wasted, on account of its coming into contact with the cold water and the 

 receiver, the surfaces of which required to be heated to its own temperature, ' 

 before the water could be expelled ; hence a tenth or a twentieth part only 

 of the steam produced could be effective ; and there would probably have 

 been a still greater loss, but for the difficulty with which heat is conducted 

 downwards in fluids. These inconveniences were in great measure avoided 

 in Newcomen's engine, where the steam was gradually introduced into a 

 cylinder, and suddenly condensed by a jet of water, so that the piston was 

 forced down with great violence by the pressure of the atmosphere, which pro- 

 duced the effective stroke: this effect was, iiowever, partly employed in rais- 

 ing a counterpoise, which descended upon the readmission of the steam, and 

 worked a forcing pump in its return, 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 afterwards 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.) 



