Steam Engine. 419 



This condensation, and consequent loss of steam produced by 

 cooling, will continue until the piston arid the portion of the cy- 

 linder situated between it and the boiler, are brought to the tem- 

 perature of the steam itself. Then, the steam preserving its elas- 

 tic form under the piston P, counterbalances entirely or partly 

 the pressure of the atmosphere upon its upper surface. The 

 excess of weight in P', acting therefore without obstacle, causes 

 A'P' to descend and AP to ascend, which tends to produce in the 

 cylinder a vacuum into which, steam rising from the boiler, con- 

 tinues to enter, until the piston P, having reached its highest point, 

 the cylinder is completely filled with steam. Having obtained 

 this limit, the steam opens the valve S and escapes, at first slowly 

 and in the form of a cloud, on account of its mixing with the air 

 and drops of water. According as the air is expelled this cur- 

 rent becomes gradually stronger and more transparent. When 

 the operator perceives that this point is attained, he turns back 

 the stop-cock, and then the whole cavity of the pump remains 

 filled with pure steam which only wants to be condensed by 

 sudden cooling in order to leave a vacuum under the piston P. 

 This condensation is effected by the introduction of cold water 

 which is made to descend from an elevated reservoir Z through 

 the tube ZRP, closed at R' by a stop-cock called the injection 

 stop-cock. Upon turning this, the cold water thrown into the 

 cylinder T7\ precipitates entirely or partly the steam contained 

 there and it flows out through the tube EOS' with the water which 

 results from this condensation ; then a vacuum being left under 

 the piston P, the pressure of the atmosphere causes it to descend. 

 This piston is again raised by the introduction of fresh steam; 

 for if, as we have supposed, the water in the boiler is kept in a 

 state of ebullition, the steam has an elastic force at least equal to 

 that of the air, and consequently its introduction under the piston 

 P is sufficient to counteract the pressure of the atmosphere ; so 

 that the excess of weight in P 7 will raise P as before. But on 

 the other hand, the steam, if heated too much, may by its elastic 

 force cause the boiler to burst. To guard against this, we adapt 

 to the top of the boiler a safety valve S", which opens outwards 

 with a known effort. When the elastic force of the steam is 

 equal or inferior to that of the external air, the valve remains 

 closed; but when it becomes equal to that of the atmosphere 

 and the resistance of the valve together, the steam escapes and 



