This formula, however, does not agree with experiment at pressures less than an atmosphere. M. de Pambour, therefore, proposes the following changes in the values of its co-efficients:—Let P express the pressure in pounds per square foot; and let
a = 10000 b = 0·4227 m = 0·00258,
and the formula will be accurate for all pressures. For pressures above two atmospheres the following values give more accuracy to the calculation:—
a = 10000 b = 1·421 m = 0·0023.
In these investigations I shall adopt the following modified formula. The symbols S and P retaining their signification, we shall have
| S = | a | ,(10.) |
| b + P |
where
a = 3875969 b = 164.
These values of a and b will be sufficiently accurate for practical purposes for all pressures, and may be used in reference to low-pressure engines of every form, as well as for high-pressure engines which work expansively.
When the pressure is not less than 30 pounds per square inch, the following values of a and b will be more accurate:—