axis OV is known as the ordinate of that point. These names are also given to the lines that measure the distances of the point from the axes OP and OV. For instance, the distance from any point to the axis OP and parallel to the axis OV is known as the abscissa, and the distance from the axis OV to the point and parallel to the axis OP is known as the ordinate.
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24. In the pressure-volume diagram as used for the gas engine, the axis OP, representing zero volume, may be located at the extreme position of the piston toward the head end of the cylinder. Since horizontal distances parallel to OV represent volumes, it is sometimes convenient to locate the axis OP at a distance from the end of the diagram that will represent the clearance volume to the same scale that L represents the volume of the cylinder. The axis V may represent either the pressure of the atmosphere or the line of no pressure, that is, absolute vacuum. The pressure of the atmosphere is that usually employed for the axis V. Taking any point, as a, Fig. 7, the ordinate of the point is the distance a′a and the abscissa of the point is the distance a″a. Similarly, the ordinate and abscissa of the point b are b′b and b″b, respectively.
25, If the curve no, Figs. 6 and 7, represents the expansion of a gas, as the burned gases in a gas engine, it is plain that the pressure will drive the piston from right to left,
