The slightest view of the subject must show, that as the piston descends, the steam that is between them will grow continually rarer and less elastic, and that both pistons will draw the beam downwards. Suppose, now, that each one had reached the bottom of its cylinder : shut the cock a, and the eduction valve at the bottom of A, and open the cocks b and d. The communication being now established between the upper and lower part of each cylinder, their pistons will be pressed equally on the upper and lower surfaces ; in this situation, therefore, nothing hinders the counter-weight from raising the pistons to the top. Suppose them arrived at the top : the cylinder B is at this time filled with steam of the ordinary density, and the cylinder A, with an equal absolute quantity of steam, but expanded into a larger space. Shut the cocks b and .4 and open the Cock a, and the eduction valve at the bottom of A, the condensation will again operate, and cause the pistons to descend ; and thus the operation may be repeated as long as steam is supplied; and one measure full of the cylinder B of ordinary steam is expended during each working stroke.
Professor Robison gave a series of elaborate and highly interesting calculations, by which, unluckily for the ingenious inventor, it was demonstrated, that the same effect only is produced in this, as in Mr. Watt's expansion engine ; and these calculations were confirmed by the practice of those which Hornblower erected. Although he made an unsuccessful application to Parliament for an extension of the term of his patent, it does not appear that his engine obtained public patronage or approbation.
In 1797 a patent was obtained by the Rev. EDWARD CARTWRIGHT, for a very ingenious contrivance, of which high expectations were formed, from the trials made of its efficacy. The nature of this invention, and the objects which Mr. Cartwright sought to obtain, will be understood by the following figure and description.
The piston b moving in the cylinder a, has its rod prolonged downwards, at the extremity of which is attached another piston d, moving in the lesser cylinder c, opening into and being a continuation of a. From the bottom of a proceeds the pipe g, terminating in the condenser, which is formed of two concentric circular vessels, between which the steam is admitted in a thin sheet, and is condensed by coming in contact with the sides of the condensing vessel kept constantly cold by being immersed in cold water. The water formed by condensation falls into the pipe e. From the bottom of the cylinder i the bent pipe is is carried into the box a, which has a float-ball o that opens and shuts the valve p, communicating with the atmosphere. A pipe arises from this box
and opens into the boiler. There is a valve placed at i opening into the cylinder c; another at a also opening upwards. The pipe a conveys steam from the boiler to the cylinder, which may be shut by the fall of the clack r. k is a valve made in the piston b. In the figure, the piston b is shown as descending b/2: elasticity of the steam flowing from the boiler through a; the piston d, being attached to the same rod, is also descending. When the piston b reaches the bottom of the cylinder a, the tail or spindle of the valve k being pressed upwards, opens the valve, and forms a communication between the upper side of the instal and the condenser; at the same moment the valve r is pressed into its seat by the descent of the cross-arm on the piston, which prevents the further admission of steam from the boiler ; this allows the piston to be drawn up to the top of the cylinder by the momentum of the fly-wheel a, in a non-resisting medium. The piston d is also drawn up to the top of e, and the valve i is raised by the condensed water and air which have accumulated in e, and in the condenser g. At the moment when the piston has reached the to of the cylinder, the valve k is pressed into its seat by the pin or tail striking the cylinder covet; and at the same time the piston b, striking the tail of the valve r, opens it; a communication is again established between the boiler and piston, and it is forced to the bottom as before. By the descent of the piston d, the water and air, which were under it in the c, being prevented from returning into the condensing cylinder by the valve under are driven np the pipe into the box n, and the water is thence conveyed into the boiler again. The air rises above the water in a, and when by its accumulation its pressure is increased, it presses the float o downwards ; this opens the valvep, and allows it to escape into the atmosphere.
The mode of condensation was theoretically considered as being liable to ob jection, but practical experiments fully confirmed the expectations of the inventor. The arrangements are very simple, and seem admirably adapted for purposes where small power is required.
About the year 1801, Mr. Joins NUNCARROW, an Englishman, then resident at Philadelphia, gave a description of an engine formed on Savery's principle,.
for raising water to turn a water wheel, which was found to possess several ad vantages beyond the very important ones of simplicity in construction and cheapness in erection.