the volume of a cylinder is equal to the product of its area and its length, any movement of the piston P toward the left will give a corresponding increase in volume. Suppose that the area of the piston is 1 square foot, and that the distance v between the piston and the cylinder head is 1 foot, then the volume in front of the piston is 1 cubic foot. Suppose that the piston were to move a distance of v' to the left. Then, if v' equals 1 foot, the distance V between the face of the piston and the end of the cylinder would be 2 feet, and the volume contained in the cylinder in front of the piston would be 2 cubic feet. Similarly, when the piston is at a distance of 112 feet from the end of the cylinder, the volume will be 112 cubic feet. Hence, the position of the pencil, measured from left to right, corresponds to the volume of the cylinder. If a piece of paper is attached to the slide f, the pencil will draw on it a continuous line, any point on which will indicate the pressure within the cylinder corresponding to a certain position of the piston and will also correspond to a certain volume in the cylinder.
22. In the diagram shown on the slide in Fig. 6, the line drawn by the pencil shows a rise of pressure from m to n while the piston P is practically stationary. This is followed by a gradual fall of pressure from n to while the piston is moving from right to left. On the return of the piston the pencil draws the line om, its position indicating a lower series of pressures than when the piston is moving toward the left, and the gradual rise of the pencil line shows that the pressure is rising gradually as the piston approaches the head end of the cylinder.
23. Fig. 7 shows the diagram of Fig. 6 drawn to a larger scale. In a diagram of this kind, the horizontal position of any point on that diagram is measured from a vertical-line, as OP, Fig. 7, called the axis of pressures. The vertical position of a point is measured from a horizontal line as OV, known as the axis of volumes. The horizontal distance of any point from the axis O P is known as the abscissa of that point, and the vertical distance of any point from the
