In 1867 at the International Exhibition at Paris a vertical atmospheric motor was to be seen working, based on the primitive principle of the gunpowder pump of De Hautefeuille. This machine was constructed by two German engineers, Otto, and Langen of Deutz near Cologne, and was a perfected form of the Barsanti and Matteucci motor invented ten years previously. The explosion of the gases in the cylinder only served to obtain a partial vacuum underneath the piston, which was therefore forced down by the excess of atmospheric pressure above it. This arrangement had one great advantage over the Lenoir and Hugon motors, it only burnt 1350 litres of gas against their 2500 or 3000 per horse-power hour, and consequently it rapidly came into favour, and the lucky inventors were able to sell no less than 5000 motors in a few years. The motor itself was very rough and had many defects: the gear-wheels rattled and made a furious noise, the igniting flame kept up a continuous roar, and above the noise of clanking machinery the explosion of the gases could be heard like a cannon going off; in fact, no one could say that the ideal of domestic motors had been attained; but as the motors constructed in 1872 only consumed 800 litres of gas per horse-power hour, rendering power produced by this means cheaper than steam, its success was assured in spite of the defects. The success of these early attempts stimulated Dr. Otto to further efforts, and in 1878 he brought out his famous gas engine, which has earned a world-wide reputation by reason of its incontestable merits. It was based on the principle explained in the De Rochas patent which we have spoken of, but Otto undoubtedly knew nothing of this patent, and his invention was perfectly independent and fresh as far as the world was concerned. The enormous success to which the new motor attained naturally led to many unscrupulous imitations, and legal proceedings were instituted in England and France. In this country the validity of Otto’s patents were upheld, but in France the De Rochas patent was for the first time brought to light, and the verdict went against him. This verdict has been attributed to malice on the part of the French judges, for at that time the French nation would have probably conceded as little as possible to a German; but whether that be so or not, we are indebted to Dr. Otto for having made the gas motor a really practical engine after many years of patient experiment and study. At the same time as the Otto engine three other motors appeared at the Exhibition of 1878: the Bisschop gas engine constructed by Mignon and Rouart, and two others by Simon and Ravel. The Simon motor, of which only a very small number were constructed, was very interesting from the economy point of view. The explosion of the mixed gas was not allowed to take place suddenly, but proceeded gradually as the piston moved forward, and the heat which in the Otto