THE STEAM-ENGINE. 



413 



united, in consequence of the principle of condensation for which he had pre- 

 viously received a patent being necessary to the projected machine. We shall 

 now describe the atmospheric engine, as first constructed by Newcomen : 



The boiler K (fig. 4) is placed over a furnace I, the flue of which winds 

 round it, so as to communicate heat to every part of the bottom of it. In the 

 top, which is hemispherical, two gauge-cocks G G' are placed, as in Savery's 

 engine, and a puppet valve V, which opens upward, and is loaded at one pound 

 per square inch ; so that when the steam produced in the boiler exceeds the 

 pressure of the atmosphere by more than one pound on the square inch, the 

 valve V is lifted, and the steam escapes through it, and continues to escape 

 until its pressure is sufficiently diminished, when the valve V again falls into 

 its seat. This valve performs the office of the safety-valve in modern engines. 



The great steam-tube is represented at S, which conducts steam from the 

 boiler to the cylinder ; and a feeding pipe T, furnished with a cock, which is 

 opened and closed at pleasure, proceeds from a cistern L to the boiler. By 

 this pipe the boiler may be replenished from the cistern, when the gauge-cock 

 G' indicates that the level has fallen below it. The cistern L is supplied with 

 hot water, by means which we shall presently explain. 



To understand the mechanism necessary to work the piston, let us consider 

 how the supply and condensation of steam must be regulated. When the 

 piston has been forced to the bottom of the cylinder by the atmospheric pres- 

 sure acting against a vacuum, in order to balance that pressure, and enable it 

 to be drawn up by the weight of the pump-rop, it is necessary to introduce 

 steam from the boiler. This is accomplished by opening the cock R in the 

 steam-pipe S. The steam being thus introduced from the boiler, its pressure 

 balances the action of the atmosphere upon the piston, which is immediately 

 drawn to the top of the cylinder by the weight of the pump-rod D. It then 

 becomes necessary to condense this steam, in order to produce a vacuum. To 

 accomplish this, the further supply of steam must be cut off, which is done by 

 closing the cock R. The supply of steam from the boiler being thus suspend- 

 ed, the application of cold water on the external surface of the cylinder becomes 

 necessary to condense the steam within it. This was done by enclosing the 

 cylinder within another, leaving a space between them.* Into this space cold 

 water was allowed to flow from a cock M placed over it, supplied by a pipe 

 from the cistern N. This cistern is supplied with water by a pump 0, which 

 is worked by the engine. 



The cold water supplied from M, having filled the space between the two 

 cylinders, abstracts the heat from the inner one ; and condensing the stream, 

 produces a vacuum, into which the piston is forced by the atmospheric pressure. 

 Preparatory to the next descent, the water which thus fills the space between 

 the cylinders, and which is warmed by the heat abstracted from the steam, 

 must be discharged, in order to give room for a fresh supply of cold water 

 from M. An aperture, furnished with a cock, is accordingly provided in the 

 bottom of the cylinder, through which the water is discharged into the cistern 

 L ; and being warm, is adapted for the supply of the boiler through T, as al- 

 ready mentioned. 



The cock R being now again opened, steam is admitted below the piston, 

 which, as before, ascends, and the descent is again accomplished by closing 

 the cock R, and opening the cock M, admitting cold water between the cylin- 

 ders, and thereby condensing the steam below the piston. 



The condensed steam, thus reduced to water, will collect in the bottom of 

 the cylinder, and resist the descent of the piston. It is therefore, necessary to 

 provide an exit for it, which is done by a valve opening outward into a tube 

 * The external cylinder is not represented in the diagram. 



