(71.)

The first experiment in which Watt attempted to [Pg125] realise, on a small scale, his conceptions, was made in the following manner. The cylinder of the engine was represented by a brass syringe A B ( fig. 20.) an inch and a third in diameter, and ten inches in length, to which a top and a bottom of tin plate was fitted. Steam was conveyed by a pipe, S, from a small boiler into the lower end of this syringe, a communication being made with the upper end of the syringe by a branch pipe D. For the greater convenience of the experiment, it was found desirable to invert the position of the cylinder, so that the steam should press the piston P upwards instead of downwards. The piston-rod R therefore was presented downwards. An eduction pipe E was also inserted in the top of the cylinder, which was carried to the condenser. The piston-rod was made hollow, or rather a hole was drilled longitudinally through it, and a valve was fitted at its lower end, to carry off the water produced by the steam, which [Pg126] would be condensed in the cylinder in the commencement of the process. The condenser used in this experiment operated without injection, the steam being condensed by the contact of cold surfaces. It consisted of two thin pipes F, G of tin, ten or twelve inches in length, and the sixth of an inch in diameter, standing beside each other perpendicularly, and communicating at the top with the eduction pipe, which was provided with a valve opening upwards. At the bottom these two pipes communicated with another tube I of about an inch in diameter, by a horizontal pipe, having in it a valve, M, opening towards I, fitted with a piston K, which served the office of the air-pump, being worked by the hand. This piston, K, had valves in it opening upwards. These condensing pipes and air-pump were immersed in a small cistern, filled with cold water. The steam was conveyed by the steam-pipe S to the bottom of the cylinder, a communication between the top and bottom of the cylinder being occasionally opened by a cock, C, placed in the branch pipe. The eduction pipe leading to the condenser also had a cock, L, by which the communication between the top of the cylinder and the condenser might be opened and closed at pleasure. In the commencement of the operation, the cock N admitting steam from the boiler, and the cock L opening a communication between the cylinder and the condenser, and the cock C opening a communication between the top and bottom of the cylinder, being all open, steam rushed from the boiler, passing through all the pipes, and filling the cylinder. A current of mixed air and steam was thus produced through the eduction pipe E, through the condensing pipes F and G, and through the air-pump I, which issued from the valve H in the eduction pipe, and from the valve in the air-pump piston, all of which opened upwards. The steam also in the cylinder passed through the hole drilled in the piston-rod, and escaped, mixed with air, through the valve in the lower end of that rod. This process was continued until all the air in the cylinder, pipes, and condenser, was blown out, and all these spaces filled with pure steam. The cocks L, C, and N, were then closed, and the atmospheric pressure closed the valve H and the valves in the air-pump piston. The cold surfaces condensing the steam in [Pg127] the pipes F and G, and in the lower part of the air-pump, a vacuum was produced in these spaces. The cock C being now closed, and the cocks L and N being open, the steam in the upper part of the cylinder rushed through the pipe E into the condenser, where it was reduced to water, so that a vacuum was left in the upper part of the cylinder. The steam from the boiler passing below the piston, pressed it upwards with such force, that it lifted a weight of eighteen pounds hung from the end of the piston-rod. When the piston reached the top of the cylinder, the cocks L and N were closed, and the cock C opened. All communication between the cylinder and the boiler, as well as between the cylinder and the condenser, were now cut off, and the steam in the cylinder circulated freely above and below the piston, by means of the open tube D. The piston, being subject to equal forces upwards and downwards, would therefore descend by its own weight, and would reach the bottom of the cylinder. The air-pump piston meanwhile being drawn up, the air and the condensed steam in the tubes F and G were drawn into the air-pump I, through the open horizontal tube at the bottom. Its return was stopped by the valve M. By another stroke of the air-pump, this water and air were drawn out through valves in the piston, which opened upwards. The cock C was now closed, and the cocks L and N opened, preparatory to another stroke of the piston. The steam in the upper part of the cylinder rushed, as before, into the tubes F and G, and was condensed by their cold surfaces, while steam from the boiler coming through the pipe S, pressed the piston upwards. The piston again ascended with the same force as before, and in the same manner the process was continually repeated.