As explained in a former paragraph the length of the vertical lines represents certain definite pressures, each inch of length representing so many pounds as per square inch, the exact amount per inch depending on the indicator spring strength or adjustment. To make this point clear, all of the indicator diagrams shown in this chapter will be provided with a scale of pressures at the left of the diagram by which the pressure at any point may be accurately measured off for practice. It should be noted that points on the curves which are above the atmospheric line represent positive pressures above the atmosphere, and that the points lying below the atmospheric line represent partial vacuums which may be expressed as being so many pounds per square inch below the atmosphere. The vacuum pressures indicate the extent of the “suction” created by the piston when drawing in a charge of air and gas. Straight vertical lines show that the increase of pressure along that line has been practically instantaneous in regard to the piston velocity, for if the pressure increased at a slow rate this line would be inclined toward the direction in which the piston was moving, as the piston would have moved a considerable distance horizontally while the pencil was moving vertically. This inclination of the vertical line gives an idea of the rate at which the pressure increases in relation to the piston speed, the greater the inclination, the slower is the rate of pressure increase. Straight horizontal lines that lie parallel to the atmospheric line denote a constant pressure or vacuum. The rate at which horizontal lines descend or incline to the atmospheric line represents the rate at which the pressure increases or decreases, in respect to the piston position (not piston velocity). A steep curve represents a rapid expansion or compression from one piston position to the next. A waving or rippling line indicates vibration due to valve chattering or explosion vibrations. A straight inclined line shows that the pressure is decreasing or increasing in direct proportion to the piston position. (36) Diagram of Four Stroke Cycle Engine. By referring to paragraph 25, Chapter III, it will be seen that the five events of suction, compression, ignition, expansion and exhaust are accomplished in four strokes, in the following order: Stroke 1. Suction—(Mixture drawn into cylinder). Stroke 2. Compression—(Mixture compressed). Stroke 3. { Ignition. { Expansion (working stroke). Stroke 4. Exhaust—(Scavenging stroke).