To stop the machine, the self-starting mechanism must first be allowed to operate, and then the supply of gas is shut off. After this the burner or electric ignition must be cut off, and the cooling water too, if it is not a circulating system. If the machine is to remain stopped for some considerable period, turn the fly-wheel round till the piston is in its most forward position; this will prevent dust and grit from getting into the cylinder, and scoring the inside when the engine is re-started. These are the general rules which must be obeyed in the maintenance of the gas engine. The attention they require is therefore small, and this quality has contributed not a little to their success. No stoker or engine-driver is required; any person can, with very little instruction, take charge of them. If the engine is properly erected by skilled workmen, and has been running sufficiently long to have arrived at a condition of regular lubrication, etc., a person in constant attendance can be dispensed with, and it is sufficient to look in occasionally and see that everything is all right, that there is a sufficient supply of oil, and that the cylinder is cool, because if it became overheated it might bind and destroy the interior. Every month a complete cleaning should be undertaken, the valve mechanism and cylinder being washed with mineral oil to dissolve the deposit of carbon and tarry substances in them. Every six months the valves should be re-adjusted, and if necessary re-ground into their seatings. The time and trouble necessary for these periodical cleanings will be amply repaid by the satisfactory working of the engine; nothing is more annoying than a breakdown due to accumulation of dirt, and to the continual postponement of the cleaning out. _Maintenance of oil engines._—These machines require the same careful treatment as gas engines, but they have in addition a carburator or a vaporizer which require special attention, and render the cylinder particularly liable to become fouled by the heavier products of the vaporized oil. We have already described the principal systems of vaporizers and carburators, and we shall only return to them to state, that for efficient working they should be cleaned out once a week. At the very outside not more than a month should elapse between the removals of incrustations and deposits of carbon. The lamps used for supplying heat to the vaporizer should also be attended to from time to time, and occasionally thoroughly cleaned out. _Ignition of the explosive mixture._—As we have already seen, several different methods exist of setting fire to the explosive gases, and each of them has its advantages and drawbacks. Ignition by an incandescent tube of iron or porcelain seems to be the most simple, and works as well as any other system; the sliding valve containing the igniting jet is somewhat more complicated.