94 
THE NATURE AND 
[sect. it. 
not to exceed about twenty-eight inches, when the force in the boiler is about 
thirty-six inches. 
149. It is obvious that the higher the force and temperature, the greater will 
be the reduction by cooling, and therefore the loss in engines of Woolf’s method 
of construction, where the steam has to make its way round the cylinders, must 
be greater, and take away much from that increase of effect arising from the use 
of high pressure steam, to gain which so much risk at the boiler is encountered. 
Of the Area of the Steam Passages. 
150. The formula for calculating the motion of steam in an engine has no 
maximum value to assist us in the choice of a proportion for applying it in 
practice; but it shows that the larger we make the aperture, the less we shall lose 
of the elastic force of the steam. On the other hand, we have shown that the 
loss of force by loss of heat is greater, the less the velocity and diameter of the 
pipe. The proportions, however, which about render the loss by the two causes 
equal have been found most convenient in practice, and therefore claim the pre 
ference. There are two rules in use, and neither of these is exactly the same as 
the theoretical one. 
151. The one is to make the diameter of the steam ways one-fifth of the 
diameter of the cylinder. This appears to be Boulton and Watt’s proportion. 
152. The other is to make the area of the passage one superficial inch for 
each horse power. 
153. The obvious intention of these rules is, that the steam should move 
with the same velocity, or require the same impelling force, in any sized engine. 
Either of them gives nearly the result, but neither of them gives it exactly. For 
the horse power in a small engine requires more steam than in a large one, and 
therefore the aperture should be greater in small engines, or less in large ones, than 
one inch area for each horse power. 
Again, engines having a short stroke move slower than those with a long one, 
and therefore should have the steam passage of a different proportion of the 
diameter according to the velocity. 
154. To render the velocity very nearly the same in all cases, we have this 
rule. 1 
1 From the equation (art. 144.) we have, when n = •00694, supposing xfv of the force to 
be lost in producing the velocity, 
AY 
U ~ 3*357 V 459 + F