Tellier proposed to use his engine for propelling omnibuses and other vehicles; but it would appear that it is too expensive and too cumbrous to be practically useful; there can, however, be very little doubt that the principle will be used with success in some new form. A patent has quite recently been taken out for such an engine in England. It will be perceived at once how the ammonia engine illustrates the law of storing up force. It originates no power of its own, but simply gives out by degrees the mechanical force which had been previously employed to change the ammonia from a gas to a liquid. Lenoir's "gas-engine" has been more successful; for, although but a few months old, it has been already largely adopted in Parisian hotels, schools, and other large establishments, for raising lifts, making ices, and even--for what is not done now-a-days by machinery?--cleaning boots. In London, it was lately exhibited in Cranbourne Street, and is now used for turning lathes and for other light work. This engine, like the ammonia-engine, is provided with an ordinary cylinder, into which coal-gas and air are admitted, under the piston, in the proportions of eleven parts of the latter to one of the former. The mixture is then exploded by the electric spark, and the remaining air, being greatly expanded, drives up the piston. When the top is reached the gas and air are again admitted, but this time above the piston, and the explosion is repeated, so that the piston is driven down again. The most ingenious part of the whole thing is the mechanism by which the electric spark is directed alternately to the upper and lower ends of the cylinder. This cannot be satisfactorily explained without a diagram, but is brought about (roughly speaking) by connecting either end of the cylinder with a semicircle of brass, which is touched by the "rotary crank" in the course of its revolution. The crank is already charged with electricity, and so communicates the electric spark to each of the semicircles in turn. The cylinder is kept plunged in water, so that there is no fear of its overheating by the constant explosions. This engine has cheapness for its main recommendation. A half-horsepower gas-engine (the commonest power made) costs, when complete, £65, and consumes twopence worth of gas per hour; while the cost of keeping the battery active is about fourpence per week. An engineer of Lyons, M. Millon, has since proposed to use, instead of coal-gas, the gases produced by passing steam over red-hot coke. These gases are found to explode rather more quickly than coal-gas, when mixed with common air, and fired by the electric spark. They will probably be found cheaper and more efficient when they can be obtained; but in many cases coal-gas will be the only material available.