proach the limit of economy as closely as it is possible to do. It is the invention of M. Simon, and was first brought to extended public notice at the Paris Exposition of 1878, where it was exhibited in sizes of from one to four horse. A diluted mixture of air and gas is used, as in the Otto, but the compression is done not in the power but in a separate cylinder. It is admitted to the cylinder by a slide-valve of similar construction to that of the Otto, but the manner of using it differs materially from that employed in the latter machine. In the Otto the entire combustible charge is introduced into the cylinder before ignition takes place, and, though the proportion of air is such that the combustion is not an explosive one, still it is completed within a time that covers but a small fraction of the stroke. In the Simon, on the other hand, the combustible mixture is introduced in small quantities that are successively inflamed, producing a gradual expansion of the gases that, along with the steam also admitted, exert a pressure upon the piston more nearly like that in the steam-engine than is attained in any other machine. The ignition is accomplished by a jet always lit, placed just inside of several thicknesses of wire gauze, to prevent the flame retreating into the combustible mixture without the cylinder. The water that is afterward admitted to the cylinder is first used to jacket it, and thus becomes slightly warm. From the jacket it is passed into a steam generator, where a portion of it is vaporized by the heat of the exhaust-gases. This steam is then admitted into the cylinder along with the combustible charge, and further heated and expanded by the ignition of the latter. The water in the steam generator circulates through the jackets of both the compression and power cylinders, first taking heat from the compression cylinder, then from the power cylinder, and reaching the generator quite warm. The heat is therefore utilized to the utmost, and as a consequence the economical result is superior to any before attained. According to M. Simon, the consumption of gas is a little less than eighteen feet an hour per horsepower. The motor occupies a little larger floor-space than the Otto, though it is of less weight, and is somewhat higher in price.
Either of these engines could doubtless be made in sizes small enough to drive simply a sewing-machine or a scroll-saw, though probably not at prices that would allow of their extended use. Only one gas-engine appears to have been so far made of such small power—that invented by M. de Bisschof, and shown in Fig. 14. It is much simpler than the above engines, but is also a much less perfect machine, though sufficiently economical for the use for which it is designed. The ordinary machine is about one man-power, and is furnished at something over a hundred dollars; a larger one of four times the power costing a hundred and ninety. It is compact, ornamental in design, and runs smoothly. No oiling is required, and, once started, it may therefore be left to itself for a considerable time. One of them, indeed, is reported to have run for forty-seven days without