SECT, vi.] CONDENSING ENGINES. 195 



3 x 13-5 x (2-3 x -12516 + -3) no.r c n- n i i e 



j - 5'9o Ibs. per circular inch, for 



the mean pressure. 



413. The velocity should be found by the rule, art. 343. and the quantity 

 of steam will be as much less than that required for an engine working at full 

 pressure, as the portion of the stroke at which the steam is cut off is less than the 

 whole stroke ; and in other respects the quantity of water, fuel, water for con- 

 densation, &c. should be determined by the rules in art. 407. 1 The counter- 

 weight will be less in the same ratio, as the pressure on the piston is less than 

 it is in a common engine. Owing to a larger sized engine being required, the 

 expansive method is not valued as it ought to be, except when the force of the 

 steam in the boiler is increased ; and this I would recommend to the extent of two 

 atmospheres, but not higher. 



414. The double engine of Eoulton and Watt. It has been already shown in 

 what the double engine differs from a single one (art. 389). The parts are 

 shown in Fig. 1. Plate v. where C is the cylinder; the steam enters at S, and 

 passes into the upper part of the cylinder at F, or into the lower part at D, as in 

 Fig. 3. ; Fig. 1. showing the piston in the state of descending, and Fig. 3. as 

 ascending. From the lower part of the cylinder in Fig. 1. the steam escapes 

 through D into the condenser B, where it is condensed by a jet of cold water, 

 which plays into it constantly ; and the uncondensed gases and water pass through 

 the valve G during the ascending stroke, and during the descending one they 

 pass from the lower to the upper side of the pump bucket, through its valves, 

 and are drawn up by the ascending stroke, and expelled at the valve Q into the 

 hot well. When the steam piston P ascends, the steam from the upper part of 

 the cylinder passes through F down the pipe E to the condenser. The steam 

 passages D and F are opened and closed by a D-slide', so called from its plan 

 resembling the letter D ; it is moved by the rod O, by tappets or other methods 

 (see Sect. vn. where the different methods are described). In small engines the 

 steam passages are frequently opened and closed by cocks, in larger ones by 

 valves or slides, the species of which, and the pistons and other parts, are described 

 in Sect. vn. 



415. The proportions of the parts for a double engine acting with full pressure. 

 When the case to which the engine is applied will admit of it, the length of the 

 cylinder should be twice its diameter (art. 329). The velocity of the piston in 

 feet per minute should be found by multiplying the square root of the length of 



1 Taking the example of art. 409. we find 22,000,000 Ibs. may be raised 1 foot by a bushel, 

 or nearly 250,000 Ibs. by 1 Ib. of coal ; and I do not think more has been actually done with low 

 pressure steam by a single engine. 



