SECT, ix.] OF THE APPLICATION OF STEAM ENGINES. 275 



Whence, if P = the mean effective force on the steam piston in Ibs. per circular 

 inch, we have 



the diameter of the steam piston in inches. 1 



And as 180 feet per minute is a very good velocity for raising water, if Q 

 be the quantity in cubic feet, 



hence, I 



/-7332AQU 

 : ( P ) 



Example. Suppose it be required to raise 80 cubic feet of water per minute 

 by a single acting engine, the mean effectual pressure of the steam being 11 Ibs. 

 per circular inch, and the lift 149 fathoms in 6 lifts. In this case 149 fathoms = 



(7^S<? v Q48 v 80 \ i i 



- ^- -)' = (5054-171)* 



= 72 inches nearly, for the diameter of the cylinder ; and (2-15 x 80)^ = 13-115 

 inches = the diameter of the pump ; the velocity being 180 feet per minute. 

 Both these diameters ought to be increased 5 per cent, for contingencies. 



DRAINAGE OF MINES. 



573. In this country the drainage of mines is a subject of vast importance. 

 It is mines which supply the means of employing steam power, and also a large 

 proportion of the materials on which that power is expended. To persons accus- 

 tomed to mines, it is seldom necessary to state those principles which should direct 

 them in the choice of engines. The absolute necessity of an economical system of 

 drainage is felt and acted upon ; and it is by comparison of annual expense, and 

 not by a comparison of the effect from a given quantity of fuel, that this economy 

 should be estimated. 



A mine engine should be simple in construction, durable in use, and made with 



1 When a given quantity of water is to be raised ; if Q be that quantity, we have 



2Q PP* 2Q P PS 



: -00518 ' a = : 34O ' ' -00518 v == -341 h ' 



2 P 

 Also, / = -|g- ; an d _ 93 ^ i hence we find, 



P - 



