In calculating the horse power of an engine, the only assistance given by the indicator is, that it provides a means of obtaining the average pressure of the steam throughout the piston stroke.
There are two methods of doing this, one by means of a planimeter or averaging instrument, and the other by means of lines called ordinates.
Fig. 3372.
The ordinates or lines are drawn at a right angle to the atmospheric line, as shown in [Fig. 3372], and each line is taken to represent the average height or length of one-half of the space between itself and the next lines.
Suppose, for example, that we require to get the area of that part of the diagram that lies between the dotted lines in the figure, and it is clear that the average height of this part of the diagram is represented by the height of the full line between them.
Any number of ordinates may be used, and the greater their number the greater the accuracy obtained. It is, however, usual to draw 10.
The end ordinates a and d, in the figure, should be only half the distance from the ends of the diagram that they are from the next ordinate, as will be seen when it is considered that the ordinate is in the middle of the space it represents.
The ordinates being drawn their lengths, are added together, and the sum so obtained is divided by the number of ordinates, which gives the average height of the ordinates.