4) Premature ignition may be caused by deposition of carbon or soot on the walls of the cylinder; the compression being too high for the fuel used; overheating of the piston, or exhaust valve, or of some poorly-jacketed part. 5) Using too much or a poor quality of lubricating oil, or a mixture too rich in fuel, causes deposition of carbon on the cylinder. 6) The use of too much cylinder oil is indicated by a blue smoke issuing from the exhaust. 7) Pre-ignition, or a bearing out of order, or the engine not being securely fastened to its foundation, causes pounding. 8) Too much water in the oil used for fuel causes white smoke to issue from the exhaust pipe. This may be caused by a leaky jacket on gasoline engines. 9) Stop the engine by shutting off the supply of fuel. Open the switch to the ignition system. Close the lubricators and oil cups, and turn off the jacket water. 10) In cold weather, drain off the jacket water to prevent freezing. 11) Always leave the engine clean and in order to start again. 12) For safety, belts and wheels should be boxed in wherever possible. Fig. 151 should be studied closely for a better understanding of the engine. =271. Generating Electricity for Homes.= Water motors, kerosene, gas and gasoline engines are the sources of power commonly used to generate electricity for private homes. A device for generating electricity is called a dynamo (Fig. 152). The electricity generated is either used directly while the engine is running, or it is stored in storage batteries. From here it is conducted thru wires and used for lighting, heating and turning motors to do work. =272. Batteries.= Batteries are used mainly where a small amount of current is needed, as on oil or gasoline engines, to make the spark to ignite the gasoline or oil, and in lighting gas and acetylene lamps, and for some door bells. There are several kinds of batteries, as liquid, dry-cell and storage. =273. Liquid Batteries.= In liquid batteries, electric current is generated by means of direct chemical action between an acid and two other substances, one more easily attacked by the acid than the other (Fig. 153), such as zinc and copper. This forms a simple cell, one form of primary battery. When the chemicals and metals in a primary battery are exhausted, they can be replaced with new metal or solution. [Illustration: FIG. 152. Electric generator.] =274. A Dry-Cell Battery.= A dry-cell is another form of battery. In these, the moisture of the acid substance is absorbed by some material like plaster-of-Paris flour or blotting paper, so that it can act on the metals or carbon in the cell and still make a cell easily transportable. The absorbed moisture in dry cells slowly evaporates, and then they become worthless. These batteries are usually thrown away after they have been used and have ceased to generate electricity.