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The central part of the link could not be used for the steam distribution, as it was necessary to enlarge it to allow for the play of the pin which was not in operation. It may be asked why the inventor used two separate pins mounted on a crosspiece on the end of the eccentric rod instead of a single pin on the centre of the rod which would have served for both forward and backward motion without requiring the centre enlargement of the link. It must be borne in mind that the raising or lowering of the eccentric rod by the reverse shaft was equivalent to a slight change in the angular advance of the eccentric. Consequently, with a link of sufficient length, to keep down the effect of the angularity it was necessary to reduce as much as possible the movement of the eccentric rod. Notwithstanding its differences the mechanism described in the patent of 1844 is in principle similar to the valve motion with which every one is to-day familiar, and which the inventor constructed as early as 1848, as is shown by a drawing taken from the records of the Brussels shops, on which appears the inscription "variable expansion; E. Walschaertsʼ system applied to Locomotive No. 98, Brussels, September 2d, 1848.”
Fig. 27, taken from this drawing, shows the valve motion as we know it to-day. For although it is true that the link and the combination lever are usually placed in a different position so as to shorten the
eccentric rod and the valve-stem, yet the design of the locomotive often requires an arrangement similar to that shown above. The system which Mr. Heusinger Von Waldegg invented in 1849, and which he applied in 1850 to 1851, differs only in a few insignificant particulars from that shown in Fig. 27. Walschaerts had therefore preceded him.