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IMPROVEMENT ON ECCENTRIC RODS. Sir—A plan has long been desired for working the sliding valves of a locomotive engine with two fixed eccentrics, (that is one to each cylin der) so as to give the lead correctly when the motion of the engine is re versed, that is to say, when the engine is working either way. There have long since been locomotive engines constructed with only two eccentrics, and so as to give the required lead to the valves, when working in either direction; but these eccentrics used to work loose upon the shaft, and when the motion of the engine was required to be changed, their situations were altered, by means of levers and catches. But before these catches could get to their proper places, the shaft was obliged to be turned, nearly half way round at least, therefore, each engine was furnished with a set of rods and levers to enable the engine inan to work each valve by hand, until the shaft came to the proper place for the catches to go together. This plan, in consequence of the tediousness in reversing the motion, its being so very liable to get out of repair, and other objections, has nearly fallen into disuse. The plan now almost universally adopted, consists of four, all of which are firmly fixed to the shaft. These eccentrics are so arranged that two of them work the valves when the engine is going in the forward direction, and the other two work the valves when the engine is going in the backward direction. The four eccentric rods are all connected to one main lever, namely, the reversing lever, and by this lever two of the eccentric rod-ends may be attached to, at the same time the other two will be detached from, the levers which work the valves. With this arrangement the starting, and the reversing, of the engine are so simple as to be performed by the greatest novice; while with the former, the engine man requires considerable practice before he can get properly into the way of starting and reversing. A plan for reversing the motion of the engine with greater ease, and for giving the lead to the valves with greater accuracy than that with four eccentrics, can hardly be desired; but it has long been the study of many ingenious persons to contrive a method from which they may obtain exactly the same result with tivo fixed eccentrics. This subject has, to my knowledge, been the cause of many experi ments, some of which have by accident arrived pretty near to the point of correctness ; but on their being performed upon a larger scale, in consequence of the persons engaged in them not being thoroughly acquainted with their ruling principles, they were deemed incorrect. There are those who have studied this subject so minutely, and made so many unsuccessful experiments, as to at last conclude it impossible to obtain this result in the manner alluded to. I have seen several ingenious diagrams intended to prove the impossibility, and I have even known attempts made to prove it impossible by geometrical de monstration. I think it needless for me to enter into the details of the valve work, but, however, I will give you a short description of the method of setting the four eccentrics, which will refresh your memory with their principles, and at the same time perhaps, serve for as good proof of the plan I am about to describe, as can readily be given. As the eccentrics, and all the other parts of the valve work, belong ing to the one cylinder, are generally the same as, but quite indepen dant of, those belonging to the other cylinder. And as each pair of eccentrics require to be set at exactly the same angle with their respective cranks, I think it will render the explanation much plainer, to only take into consideration the two eccentrics belonging to one cylinder, namely, one for the forward, and the other for the backward motion. Suppose A B C D, fig. 1, to be a circle described by the crank, a, the lever to which the eccentric rods are to be attached, E C, a line drawn through the centres of the cylinder, end of the lever, and the crank axle, and B D another line also drawn through the centre of the crank axle, but perpendicular to E C. Suppose it to be at C. Now, when the crank is in this situation, the piston will, of course, be at the end of the cylinder; and the lead is generally considered as the dis tance the valve has moved from the middle of its stroke, or as the distance it is open, when the piston is in this situation. To give this lead, when the engine is working in the direction shown by the arrow F, the eccentric must be set about c; and the perpendicular distance from the line B D to c, is the quantity of lead in the eccentric. Now, when the rod belonging to c, namely, the eccentric rod, is attached to a, tne valve will have the lead for working the engine in the direction shown by F, and it will continue to open until the crank arrives at G. But if the crank be turned in the direction shown by H, the eccentric will cause the valve to move in the wrong direction, and, consequently, allow the steam to act contrary to the motion of the piston; therefore, another eccentric e, is furnished, which is set at exactly the same angle with the crank as c, but on the opposite side. Both of the eccentric rod ends are connected with the reversing lever, as I have before ob served, by which they maybe detached from, and attached to the lever a, at pleasure. It will be seen, by a little attention to the figure, that the changing of the eccentric rods, when the crank is at C, will pro duce no alteration in the position of the valve, neither is it necessary it should, because the piston is then at the end of its stroke, and, although the crank be required to turn in the other direction, the steam will still be required to act upon the same side of the piston, jg Let us now suppose the crank to be at B, the eccentrics will now be at/, g, and the piston about the middle of the cylinder. When the engine is intended to work in the direction of F, the rod belonging to / must be attached to the lever, which will cause it to stand at h, and consequently the valve will be wide open, with the exception of the little difference caused by the lead. To reverse the motion, that is to say, to set the valve for working the engine in the other direction, the valve must be made to slide so as to open to the same extent, to allow the steam to act upon the contrary side of the piston. This is accomplished by the reversing lever, which detaches the rod belong ing/, and attaches that belonging to g, which, by means of its forked end, draws the. lever from li to i, and consequently causes the steam to act on the other side of, and force back, the piston. By a little attention it may be seen that, while the crank is in any point of its revolution, the changing of the eccentric rods will produce that alteration, in the position of the valve, required to reverse the motion of the engine; therefore, I think the two points, in which we have supposed the crank, will be sufficient to explain the manner in which the lead is effected, and the motion reversed by the two fixed eccentrics to each valve. I shall now proceed to explain the principles of a plan for giving the lead to the valves, and reversing the motion of a locomotive en gine, with two fixed eccentrics, instead of four. In the following ex planation, for the same reason as in the foregoing, I shall only speak of the valve, &c., belonging to one cylinder. Suppose (as in fig. 1,) the circle A B C D, fig. 2, to be described by the crank, E C, a line drawn through the centres of the cylinder, and crank axle, and B D to be drawn perpendicular to E C. Suppose the crank to be at C, and the eccentric at e. After having determined the quantity of lead to be given by the eccentric, draw the lines F G, and H I, at the same angles with the crank, as you would set the eccen trics in fig. 1, to the same quantity of lead. Then draw the line J K, perpendicular to H I, and that encl of the lever to which the eccentric rod is attached w'hen the engine is working in the direction of L,must come in this line; supposing the valve to be worked from the lever M. By a little attention it will be perceived that, by setting the end of the lever in this situation, the valve will have the same quantity of lead, as it would if the lever and eccentric were set as in fig. 1. To cause the engine to be right for working in the contrary direction, no altera tion is necessary in the situation of the valve; still it would not do to let the eccentric rod remain attached to s, therefore, I introduce another lever c, the end of which comes into the line N O, which is drawn perpendicular to F G, and, by means of the reversing lever, I detach the eccentric rod from s, and attach it to v, which will still C
