I.H.P. = 2 × S × R × A × P/33,000.
Where S = stroke in feet.
R = revolutions per minute.
A = area of piston in inches.
P = mean pressure in lb. exerted per sq. in. on the piston.
The only difficulty is the mean effective pressure; this can be found approximately by the following rule and accompanying table.
Table VI.
| Cut-off | Constant | Cut-off | Constant | Cut-off | Constant |
| 1/6 | ·566 | 3/8 | ·771 | 2/3 | ·917 |
| 1/5 | ·603 | ·4 | ·789 | ·7 | ·926 |
| ¼ | ·659 | ½ | ·847 | ¾ | ·937 |
| ·3 | ·708 | ·6 | ·895 | ·8 | ·944 |
| 1/3 | ·743 | 5/8 | ·904 | 7/8 | ·951 |
Rule.—"Add 14·7 to gauge pressure of boiler, this giving 'absolute steam pressure,' multiply this sum by the number opposite the fraction representing the point of cut-off in the cylinder in accompanying table. Subtract 17 from the product and multiply the remainder by 0·9. The result will be very nearly the M.E.P." (R.M. de Vignier.)
APPENDIX A.
