RULES FOR CALCULATING THE STEAM ENGINE. 



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MULTIPLIER for MEAN PRESSURE of Steam worked EXPANSIVELY. 



Example. Suppose the steam to enter the cylinders at a pressure of 50 fts. on the square 

 inch, and to be shut off after the piston has moved 16 inches ; and let the length of 

 stroke be 4 feet 6 inches or 54 inches. 



16) 54 (3-375, relative expansion. 



48 



60 



48 



120 



112 



80 

 80 



The relative expansion falls between 3'3 and 3'4. Referring to the table, the multiplier for 

 3'3 is 0*6648 ; and in passing to 3'4 we see that it decreases by the printed difference 108. 

 Hence, multiplying 108 by '75 and subtracting the product 81 from 0'6648, the remainder 

 0-6567 is the multiplier answering to 3'375. Therefore, multiply 0-6567 by 50 16s. and we 

 have 32-83 Ibs. per square inch, the mean pressure required. 



In practice a small extra portion of steam will occupy the clearance of the cylinder. The 

 power of this portion is obviously lost at the beginning of the stroke, but when the steam is 

 shut off, it will evidently afford some slight assistance in the expansion, since a greater volume 

 of steam must lose less pressure in expanding through the same distance. The additional aid 

 thus derived is, however, extremely small in respect of the portion of steam expended ; and as 

 the quantity of clearance is generally very small (the smaller the better) the effect of it is dis- 

 regarded in the above rule. It would be of little avail to include the effect of the clearance of 

 the cylinder, when .the influence of other causes, such as friction, waste, &c., which vary in 

 different engines, can be but imperfectly estimated. In cases, if there be any, where the 

 accuracy of information on these points is such as to be relied upon, the following rule may be 

 adopted. 



RULE. To the distance moved by the piston before the steam is cut off, and to the actual 



