Page:Cassier's Magazine Volume XV.djvu/205

Rh each end, or "over-opened," as we should say. It is specially mentioned that "this distance that the slide moves beyond the port is called the travel."

The author also mentions, almost apologetically, that the total power is



diminished a little on account of the steam being shut off before the end of the piston's stroke, "the extent of this action being, however, very limited, as the piston is less than ¼ in. from the end of its stroke when the steam is shut off."

It is somewhat strange that the writer, and more so that the eminent reviser of this description, after so minutely detailing the many and obvious disadvantages entailed by keeping on the steam during what was practically the whole stroke of the piston, should, in his very next paragraph, disclose the remedy, but apparently without any real knowledge of the importance of his casual remark that "in stationary and marine condensing engines the steam has usually very little or no lead, but it is shut off at two-thirds or three-quarters of the stroke, giving a great amount of expansive action; and the eduction has a great deal of lead, the port being nearly full open at the commencement of each stroke."

Had these conditions been fulfilled in the engine before us, or, in other words, had lap been added to the slide valves, a very much better result would have been attained. It is evident from the description that the principal object of the designer of this engine was to get as much steam into the cylinder as possible, leaving its egress to take care of itself, with the result, of course, that the cylinder became choked or gorged with steam, which at anything above a very slow speed had to be actually forced or propelled out by the piston.

It is not at all improbable that the wheels would have revolved more quickly had the cylinder-cocks been kept fully open to relieve the congestion. The consumption of fuel in proportion to the actual work done must have been enormous under these conditions, particularly as the gab-motion with which this engine was fitted afforded no means of shortening the stroke of the valve so as to work with some degree of expansion. The combined reversing and expansion gear formed by the Stephenson-Howe link-motion was not introduced till 1843. As a matter of fact, the negative or back-pressure in this engine "amounted to 30 or 40 per cent. of the positive pressure of the steam upon the piston when the engine is running very fast,