Another type of water motor consists of cups or fans on the rim of a wheel. As the water flows over the wheel, it pushes it around, thus giving it power to do work provided there is enough pressure behind the water (Fig. 150-_a_). CHAPTER XXXVIII ENGINES; MOTORS AND BATTERIES; FUELS =267. Gasoline Engines.= A gasoline engine (Fig. 151) should be operated out of doors or in a well-ventilated room, except in cases where the exhaust pipe is carried thru the wall of the building to the outside. The fumes may cause illness, or even death, to any one staying in the room. A gasoline engine should be mounted on a substantial base of concrete or heavy timbers, or on a well-built truck, and should be put in good order before the woman or girl begins to use it. The engine must be level. If more than one device is attached to it, be sure to use the right pulleys on the engine and the machine to be operated. An engine is usually equipped with pulleys of two or more sizes. The size of the wheel on the washing machine or vacuum cleaner must be of a size to make the desired number of revolutions per minute. =268. Figuring Speed of Pulleys.= For example, if the speed of the engine is 425 revolutions per minute and the diameter of the pulley on the engine is 12 inches, and the machine is to be run at 150 revolutions per minute, have a pulley on the machine of a diameter which equals 425 times 12, or 5,100 divided by 150, or 34 inches. It would be more convenient to have a smaller pulley on this machine. Since there is a smaller wheel on the engine which, we will say, is 6 inches in diameter, put the belt on the smaller wheel, and then a wheel only 17 inches in diameter will be needed on the machine. [Illustration: FIG. 151. Sectional view of gasoline engine.] =269. Operating the Engine.= One person should be responsible for the care of an engine. Starting the engine is usually too heavy work for most women. Since a man usually starts a gas engine which the women are to use, it is more important that they know how to stop the engine and to recognize when it is not running properly. A cold engine can be started easier if warmed with hot water. Running an engine which is out of order may damage it seriously. Have some one show you how to operate your engine. Stop it when not running properly. =270. Points in Caring for Engine.= The following are points to keep in mind when operating an internal combustion engine: 1) Black smoke issuing from the exhaust pipe means there is not enough air in proportion to fuel. 2) When an engine misses more explosions than it should, or backfires, the cause is likely to be too much air in the fuel. 3) If the mixture of fuel and air is in the proper proportion, but there is too little of it, the engine will have no power.