_Ragot petroleum motor_ (Figs. 37, 38).—This engine, like the preceding ones, uses heavy petroleum and particularly schistic oil, which has a flashing-point above 77° Fahrenheit, and which may be bought very much more cheaply than naphtha, benzoline, or other artificial products of petroleum. Several different patterns of the engines are on the market, of which certain ones are of great value for electric lighting purposes, not only because of their steady running, but also because, by burning cheap oil, they allow the cost per horse-power to be reduced to a minimum. One high-speed pattern has been especially designed for coupling direct to dynamos, forming a very neat and compact plant. The latest developments in the construction of these engines tend to the simplification of the parts, and they are now so free from unnecessary mechanism, that persons with little or no knowledge of mechanics are quite capable of looking after them, and even taking them to pieces and putting them together again if a breakdown should occur, which is unlikely. [Illustration: FIGS. 37, 38.—Sections of the Ragot Petroleum Engine.] By a special arrangement in the carburator all constituents of the petroleum which might foul the cylinder are removed before the oil is allowed to enter the cylinder, and therefore it seldom requires cleaning out. The carburator (Fig. 39) is very ingeniously arranged; it consists of a cast-iron cylinder smooth on the inside, but provided with a spiral rib on its outer surface, and heated by an oil lamp. The spiral rib is itself enclosed by a covering, so that it forms a spiral tube. The petroleum is allowed to enter at the top, and gradually winds its way down, passing over a continually warmer and warmer surface as it approaches the lamp. The more volatile portions of the oil are therefore vaporized first, and the heavier oil passes on and is not vaporized till near the base, where the temperature is high enough to gasify the heavy residue. The air is heated in a jacket surrounding the base of the spiral, and is afterwards mixed with the oil vapour.