[Footnote 12: The external cylinder is not represented in the diagram.]
The cold water supplied from M, having filled the space between the two cylinders, abstracts the heat from the inner one; and condensing the steam, produces a vacuum, into which the piston is immediately 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 it has 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 already 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 opening the cock M, admitting cold water between the cylinders, 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 _outwards_ into a tube which leads to the feeding cistern L, into which the condensed steam is driven.
That the piston should continue to be air-tight, it was necessary to keep a constant supply of water over it; this was done by a cock similar to M, which allowed water to flow from the pipe M on the piston.
(40.) Soon after the first construction of these engines, an accidental circumstance suggested to Newcomen a much better method of condensation than the effusion of cold water on the external surface of the cylinder. An engine was observed to work several strokes with unusual rapidity, and without the regular supply of the condensing water. Upon examining the piston, a hole was found in it, through which the water, which was poured on to keep it air-tight, flowed, and instantly condensed the steam under it.
On this suggestion Newcomen abandoned the external cylinder, and introduced a pipe H furnished with a cock Q into the bottom of the cylinder, so that on turning the cock the pressure of the water in the pipe H, from the level of the water in the cistern N, would force the water to rise as a jet into the cylinder, and would instantly condense the steam. This method of condensing by a jet formed a very important improvement in the engine, and is the method still used.