The pressure of the gases is raised during compression to about four atmospheres, and immediately after the explosion it rises to about 180 lbs. on the square inch; during the exhaust it averages about 10 lbs. per square inch. Ignition is obtained by a tube heated to a bright red incandescence by a Bunsen flame. At the right moment a valve is opened, placing the explosive mixture in contact with it and causing the explosion. When starting the machine the ignition is retarded or takes place a little after passing the dead point, so that the machine cannot start the wrong way by mistake. Two or three other features call for special notice, especially the device for lubricating the cylinder. In the illustration (Fig. 14) will be seen a small bell-shaped receptacle. This vessel contains oil, and also a small crank inside driven by a belt off the valve-shaft. As this crank rotates it dips into the oil at the bottom of the vessel, and at the top of its path it wipes off the oil which it has gathered on to a cup which allows it to flow into the cylinder. The water jacket is cast separately from the cylinder, and not, as in many engines, in one piece with it. There is an advantage in this, because there is less likelihood of flaws or blow-holes in the cylinder wall passing unobserved when the engine is leaving the maker’s hands. The governor in some of the sizes is centrifugal, and in others of the inertia type similar to that described in the Fielding gas engine. _Pygmée motor (Lefebvre)._—This motor, shown in Fig. 16, gives one the idea of solidity and compactness. It possesses the peculiar property of working equally well in any position, either horizontal or vertical. This is due to the fact that it is particularly well balanced, and when running does not vibrate at all. Easily started, these engines run at a very constant speed, and their power in relation to their size is truly remarkable, hence the name Pygmée. They have been especially designed for self-propelled vehicles, and are not affected by the worst running conditions, such as inclement weather or bad roads. In this type they are constructed with two cylinders in order to obtain a more constant torque, and also have an arrangement by which the speed can be changed. For stationary purposes the motor is mounted on a cast-iron stand (Fig. 16). In virtue of their exceedingly small dimensions and reduced weight they are specially suitable where small-power motors are required for home industries or small workshops, and also for driving private electric installations and pumping water. They have economically replaced steam engines in agricultural operations, both on a large and small scale. The working parts being entirely enclosed they stand a good deal of rough usage, and will work in positions in which other motors would be useless. Where it is necessary to bring the motor to the work it is required to perform they are bolted to a carriage instead of a cast-iron base.