THE STEAM-ENGINE. 



423 



surface below its own temperature, will not be condensed, and therefore will 

 immediately cause the piston to rise, and the piston will have attained the top 

 of the cylinder when as much steam shall have been supplied by the boiler as 

 will fill the cylinder. When this has taken place, suppose the communication 

 with the boiler cut off, and the cock D once more opened ; the steam will 

 again rush through the pipe S into the vessel C, where encountering the cold 

 surface and the jet of cold water, it will be condensed, and the vacuum, as be- 

 fore, will be produced in the cylinder A B ; that cylinder still maintaining its 

 temperature, the piston will again descend, and so the process may be continued. 



Having carried the invention to this point, Watt saw that the vessel C would 

 gradually become heated by the steam which would be continually condensed 

 in it. To prevent this, as well as to supply a constant jet of cold water, he 

 proposed to keep the vessel C submerged in a cistern of cold water, from which 

 a pipe should conduct a jet to play within the vessel, so as to condense the 

 steam as it would pass from the cylinder. 



But here a difficulty presented itself, against which it was necessary to 

 provide. The cold water admitted through the jet to condense the steam, 

 mixed with the condensed steam itself, would gradually collect in the vessel 

 C, and at length choke it. To prevent this, Watt proposed to put the vessel 

 C in communication with a pump F, which might be wrought by the engine 

 itself, and by which the water, which would collect in the bottom of the vessel 

 C, would be constantly drawn off. This pump would be evidently rendered 

 the more necessary, since more or less atmospheric air, always combined with 

 water in its common state, would enter the vessel C by the condensing jet. 

 This air would be disengaged in the vessel C by the heat of the steam con- 

 densed therein ; and it would rise through the tube S, and vitiate the vacuum 

 in the cylinder ; an effect which would be rendered the more injurious, inas- 

 much as, unlike steam, this elastic fluid would be incapable of being condensed 

 by cold. The pump F, therefore, by which Watt proposed to draw off the 

 water from the vessel C, might also be made to draw off the air, or the princi- 

 pal part of it. 



The vessel C was subsequently called a condenser ; and, from the circum- 

 stances just adverted to, the pump F has been called the air-pump. 



These — namely, the cylinder, the condenser, and the air-pump — were the 

 three principal parts in the invention, as it first presented itself to the mind of 

 Watt — and even before it was reduced to a model, or submitted to experiment. 

 But, in addition to these, other two improvements offered themselves in the 

 very first stage of its progress. 



In the atmospheric engine, the piston was maintained steam-tight in the 

 cylinder by supplying a stream of cold water above it, by which the small 

 interstices between the piston and cylinder would be stopped. It is evident 

 that the effect of this water as the piston descended would be to cool the cyl- 

 inder, besides which any portion of it which might pass between the piston 

 and cylinder and which would pass below the piston, would boil the moment 

 it would fall into the cylinder, which itself would be maintained at the boiling 

 temperature. This water, therefore, would produce steam, the pressure of 

 which would resist the descent of the piston. 



Watt perceived, that even though this inconvenience were removed by the 

 use of oil or tallow upon the piston, still, that as the piston would descend in 

 the cylinder, the cold atmosphere would follow it ; and would, to a certain 

 extent, lower the temperature of the cylinder. On the next ascent of the pis- 

 ton, this temperature would have to be again raised to 212° by the steam 

 coming from the boiler, and would entail upon the machine a proportionate 

 \ waste of power. 



