=326. The internal combustion engine.=—In the ordinary power plant there are two units, the boiler in which steam is generated, and the engine in which the steam is put to work. An internal combustion engine is designed to burn the fuel in the engine itself. This is practicable when the fuel is a gas or a liquid whose vapor will unite with the oxygen of the air to form an explosive mixture. The internal combustion engine offers several advantages over the steam power plant: it uses more effectively the heat that is generated, it is less bulky, it is more easily tended. In spite of characteristic disadvantages, particularly the need of an auxiliary starter, and restricted flexibility as regards speed and power, the internal combustion engine has proved indispensable. In large units it serves the steel mills, which put the waste gases from their blast furnaces to work, and in its application to the automobile it has effected a revolution in transportation and travel. Although in the manufactures of the United States in 1914 the internal combustion engine still accounted for less than 5% of the total horsepower, the aggregate horsepower of the gasoline engines of automobiles has since that time considerably exceeded the total horsepower from all sources employed in manufacturing industry. QUESTIONS AND TOPICS 1. Do not attempt to remember _any_ of the figures in sect. 316, unless possibly the first two of the last line. Prepare a graphic chart in the following way. Lay off the time periods on the horizontal line at the bottom of your paper, and on the perpendicular, near the right-hand margin, lay off the figures of the last line of the table. This will insure space in the chart for all the lines. Divide the perpendicular into, say, forty units. Each unit may then be made to represent: 1,000 million dollars of value; $1 per capita; 40 million tons of coal; 2 million tons of iron. Indicate the figures for 1913 on the perpendicular (commerce 40, per capita commerce 24, coal 36, iron 39); and perform the same operation for the figures on perpendiculars above each of the other dates. Use for each item a characteristic mark, (cross, circle, triangle, square), which will enable you to distinguish it from the others. Then unite the marks of each kind by a curved or crooked line. Choose a characteristic form of line (dotted, wavy, or colored) for each item. If the chart be made on a large scale and with sufficient neatness, later tables of statistics (development of railroads, trade of particular countries, etc.), can be entered upon it. With regard to each one of the items: when was the increase (measured by the slope of the line) greatest? When least? What relation is apparent in the increase of different items? Many of the questions suggested by a study of the figures will be treated in later sections.