And, as to the wheel A itself, I have not represented it in the very form I should wish it to have, because it can be sufficiently described in words. I should cast this wheel (if made at all in metal) as a shell, the outside of which would be what is really seen in the [figure] (at A), and the rim would have in it mortices, like those which are made for iron wheels destined to receive wooden cogs, and geer with cogs of iron. In fact, this would become a wooden-toothed-wheel, with its teeth roughly formed and placed, so as to occasion a small expence, and to be easily changed, when worn away by the friction on the track-way. Thus would, I am persuaded, a very moderate weight in the wheel, and as moderate a pressure from the braces k l, connect the wheel with the road enough to produce the desired effect, with a trifling loss of the power employed. And thus might we navigate a narrow canal, with a great saving of expence; not to mention that other advantage of avoiding entirely that injury to the banks, which must attend every system of propelling the boats, founded on the agitation of it’s waters.
OF
A MACHINE,
For working, swiftly, the Slide-valves of Steam-engines.
The Slide-valve is an excellent substitute for the hand-geering of steam-engines, from the simplicity of form which it introduces, and the certainty of it’s recurring effects. But it is, I believe deservedly, reproached with being too sluggish in it’s operation, at the very moment when activity would be most desirable—namely, at the beginning of the strokes; insomuch, say some, that the power of the engine is materially lessened by it. The fact is, that the excentric (usually placed on the crank-shaft) is almost always moving, and with it the slide-valves also; which thus open by slow degrees, when they should open by rapid ones.
Without discussing the question further, I cannot refrain from introducing this application of the principle of my Parallel Motion, given in [page 237]; which appears to me greatly calculated to obviate these difficulties; and thus to leave the slide-valve in possession of all it’s own advantages, with the addition of those which have hitherto belonged exclusively to the Hand-geering System.
I have represented this Mechanism in [figs. 5 and 6], [Plate 31]: where A B shew the crank-shaft of a steam-engine, working by means of slide-valves, the place of the excentric being at a b, in a line with the pulling-bar e f. Instead, then, of the usual connecting frame between the excentric at a b, and the valve-lever at g, I use for the above purpose, a lever e f terminated by an arc o, furnished (in the present instance) with five teeth, and connected by the joint e with the valve-lever g, in the usual manner. In the arc, which terminates this lever to the right, are the five teeth above-mentioned; and, they geer in the ten teeth of the wheel c d, which will be seen (in [fig. 6]) to be on the same shaft with the spur-wheel m, itself driven by the spur-wheel n, of twice the diameter. This wheel c d, therefore, makes two revolutions for one of the crank-shaft: and, supposing it to turn in the direction of the arrow, it will first of all draw upward the arc o, producing no effect on the valve-lever at g; but, when the tooth r is arrived at p (the tooth p being then arrived at the entrance of the curve q), the wheel c d will begin to draw the arc o along with it, round it’s own centre; and, the teeth of the arc being kept in it’s teeth by the similar curve q, the valve-bar will be drawn from g to h, in the course of one quarter of a revolution of the crank-shaft A B. But, now, the tooth r of the arc o will be found at s: and, therefore, the further revolution of the wheel c d will carry the arc o downward toward t, until the tooth r has reached the point t; that is, until the wheel c d has made another half-revolution, and the shaft A B another quarter; when, as before, the arc o, conducted by the curve t r, will again drive back the lever e f, till it comes into it’s present position: after which, their motions will be regularly continued. It is, then, evident, that the slide-valves are thus opened and shut, each during one quarter of a turn of the crank-shaft A B; and thus they remain stationary during another quarter, and that, in two positions of said shaft diametrically opposite to each other. And thus have we a simple mean, adaptable to every engine, of giving it much of the advantage of the hand-geering system, while preserving all that of the slide-valve principle. And, were it desired to lengthen the interregnum of the opening motion, it would be done by making the wheel c d smaller, and the ratio of n to m (see [fig. 6]) larger in the same proportion.
I observe here, however, that care should be taken not to make the valve motions too rapid, nor the intervals between them too long; for, I consider one of the best properties of this motion to be, that it acts like an excentric; that is, slowly at first, most rapidly afterwards, and finishes as slowly as it began; which is a precious quality in all reciprocating machines.