A very simple and at the same time ingenious mode of illustrating the operations of an atmospheric steam-engine will be found in the annexed apparatus, suggested by Professor Brande, and employed in his lectures at the London Institution.
The glass tube and bulb b is shewn with its piston a, the rod being hollow and closed by a screw c. If steam be generated by the spirit lamp d, the air will speedily be expelled, and after this is effected, the screw c may be closed, and a working stroke produced by artificial condensation.
We come now to a new and distinct era in the history of this important invention, and in noticing the labours of Mr. Watt, we may almost speak of his engine as the gigantic offspring of a hand giving birth to an automaton, no less powerful than that of the fabled enchanters of the olden time.
Mr. Watt's first great improvement in the engine of Newcomen may be best understood by reference to the annexed diagram, in which a represents the cylinder, and b its plug or piston made to fit air-tight. The pipe d is furnished with a stop-cock, by means of which the elastic vapour is occasionally admitted.—A similar pipe, furnished with a stop-cock at f, passes from the other side of the cylinder, and enters the vessel g; e being the reservoir to contain water.
If we now suppose the piston at the bottom of the cylinder, and steam admitted by the pipe d, its expansive force will elevate the piston, and when the air is expelled, the whole internal cavity of the tube will be filled with condensible vapour. On closing the steam-cock, and opening that connected with the vessel g, a portion of the vapour will immediately expand itself, and coming in contact with the cold sides of the vessel, a portion of its heat must be absorbed by the water at e. A new portion of steam then descends, and is also condensed, and indeed the same process continues till the whole of the steam is drawn from the tube. A vacuum being thus formed, the pressure of the atmosphere will preponderate, and the piston rod be depressed to the bottom of the tube. On closing the stop-cock f, a new supply of steam may be admitted by the other pipe, and after raising the piston, the process of condensation may be readily repeated.
The advantages that arise from this mode of forming a vacuum are very considerable, not the least important of which, is a saving of nearly half the fuel.
In the old engine, the condensing water must reduce the temperature of the internal surface of the cylinder to that of the atmosphere, before a vacuum could be produced, and when the condensing water was applied more sparingly, the elastic vapour remaining in the cylinder was found to materially reduce the pressure of the air operating above. From this it will be seen that the great advantage of Mr. Watt's apparatus consists in performing the condensation in a separate vessel, so that the cylinder is always preserved at the temperature of boiling water.