Electric ignition, if effected by a magneto generator driven by the engine, is certain in its operation, but it is awkward at starting. For petroleum and carburetted air motors batteries both primary and secondary are better, especially when the motors are used for propelling vehicles. Looked at from the economy point of view, it is very hard to decide which is the best system of ignition: red-hot tubes require gas, accumulators require electric energy to be put into them, magneto machines are expensive and use up power, in fact, they are all about the same as far as cost is concerned. In France electric ignition is in much greater vogue than in this country, where hot tubes reign supreme. The electric system requires a few words concerning batteries and induction coil. If primary batteries are used the chromic acid type is the best. These are manufactured in a large number of varieties; some of the French makers, Trouvé, Guérot, Radiguet, and others, have paid special attention to their application to gas and petroleum engines. The induction coil should have a rather thicker secondary winding than usual; it is necessary to have as hot a spark as possible to ignite the gas, and this is not obtained if the spark is of exaggerated length. [Illustration: FIG. 51.—Section of the Simplex Gas-producing Plant.] The spark-gap also requires occasional attending to. It consists usually of a porcelain rod with two points between which the sparks pass, either intermittently or governed by an automatic switch, or else continuously. In either case the points often become coated with a deposit of carbon, which decreases their efficiency. They must therefore be wiped clean about once a week. _Maintenance of engines using poor gas._—These engines differ very little from the ordinary coal-gas engine. The arrangement of valves which will do for the one is also suitable for the other. We shall therefore only give a few hints on the management of the gas-producing plants which forms their only point of difference; to do this we shall consider a “Simplex” plant. The firm of Matter et Cie. have received many letters from other firms to whom they have supplied their apparatus, replacing existing steam engines, in which they state that the whole management and maintenance of their new source of power is much more simple than the old. This is not to be wondered at, because the whole plant is perfectly automatic. There are no pressure or water gauges to attend to, because no more gas is ever produced than is absolutely wanted. A steam boiler may require to be stoked once every quarter of an hour, whilst a poor gas generator only requires charging about once every six hours, according to demand for power. The Simplex motor itself requires very little attention, and all bearing surfaces are made as large as possible to avoid the risk of heating.