
MECHANICAL ACTION OF HEAT. 181 
(33.) of this section, the volume V, of unity of weight of steam corresponding to 
the maximum pressure P,. Then Equation (48.) gives the space traversed by the 
piston in unity of time, which, being multiplied by the resistance R per unit of 
area of piston, gives the gross effect of the engine. 
(27.) If, on the other hand, the space traversed by the piston in unity of time 
is fixed, Equation (48.) gives the means of determining, from the evaporating 
power of the boiler W, either the volume V, of unity of weight of steam required 
to work the engine at the given velocity with a given expansion, or the expansion 
$ proper to enable steam of a given initial density to work the engine at the given 
velocity. The initial pressure P, being then determined from the volume V,, the 
resistance which the engine is capable of overcoming with the given velocity is to 
be calculated by means of Equation (51.) 
(28.) This calculation involves the determination of the pressure P, from the 
volume V, of unity of weight of steam at saturation, which can only be done by 
approximation. The following formula will be found useful for this purpose :— 
12 
Pj=o () zane sh $60 .1625 
where zw represents the pressure of one atmosphere, V, the volume of steam of 
saturation at that pressure (being 1696 times the volume of water at 4°:1 cent., 
or 27-136 cubic feet per pound avoirdupois), and V, the volume of steam of satu- 
ration at the pressure P,. This formula is only applicable between the pressures 
of one and eight atmospheres: that is to say, when the volume of steam is not 
greater than 27 cubic feet per pound, nor less than 4, and the temperature not 
lower than 100° centigrade, nor higher than 171° centigrade (which correspond to 
212° and 340° Fahrenheit). 
The greatest error in computing the pressure by means of this formula is 
about #5 of an atmosphere, and occurs at the pressure of four atmospheres, so 
that it is st) of the whole pressure. This is sufficiently accurate for practice, in 
calculating the power of steam-engines; but should a more accurate result be 
required, the approximate value of the pressure may be used to calculate the 
temperature by means of Equation XV.; and the temperature thus determined 
(which will be correct to ¢ of a centigrade degree) may then be used in conjunc- 
tion with the volume to compute a corrected value of the pressure, according to 
Equation (38.) The pressure, as thus ascertained, will be correct to s¢55 of its 
amount, which may be considered the preatest degree of accuracy attainable. 
The most convenient and expeditious mode, however, of computing the pres- 
sure from the volume, or vice versd, is by interpolation from the table given in the 
Appendix to this paper. 
(29.) The resistance denoted by R may be divided into two parts; that which 
arises from the useful work performed, and that which is independent of it, being, 
