94 THE NATURE AND ' [ SEC T. n. 



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 -^ part of the force to 

 be lost in producing the velocity, 



_ AV _ 

 ~ 3-357 V 459 + i 



