200 OF THE CONSTRUCTION OF [SECT. vi. 



passage c, enters the large cylinder at d, while the steam passes to the condenser 

 through e. When the motion is reversed by the slide being moved till the parts 

 are on the other side of the passages, then similar motions take place in the reverse 

 directions, and the vapour passes through / down a pipe to the condenser. Thus 

 the whole apparatus is reduced to a slide box, the rod of which has only one 

 motion for each stroke ; and though it is here shown between the cylinders for 

 convenience, it may be placed in the angle they form when close to each other. 



426. The proportions of combined engines. The smaller cylinder should have 

 the same proportions as for a non-condensing engine working with steam of the 

 same force, (art. 366.) and the loss of force must be the same, that is 0*4 of the 

 force of the steam in the boiler. 



The loss of force at the piston of the large cylinder, when its power is 1 will be, 



1. By the cooling in the cylinder and pipes - -016 



2. By the friction of the piston - -125 



3. By the force necessary to expel the steam through the passages '007 



4. By the power required to work the air pump - -050 



198 



Consequently, '6 x -198 = '1188 = the portion of the whole power, which added to 

 the loss in the small cylinder, the total loss is '1188 -*- -4 = -5188, or -52 nearly. 

 Hence, if / denote the whole force of the steam in the boiler, 3-7 the resistance of 

 the uncondensed steam, and n the times the capacity of the large cylinder is to 

 exceed the small one, we have 



n = 



52 / + 3-7 

 If for example the force of the steam in the boiler be 120 inches of mercury, then 



120 _ 1 .ag. 



(52 x 120) + 3-7 " 



that is, the large cylinder should be 1'82 times the capacity of the small one ; if it 

 be larger, a loss of effect must necessarily ensue. 



427. The power of a combined cylinder engine is easily ascertained from 

 the investigation, art. 382. by substituting the proper constant numbers. The 

 resulting rule for the mean pressure, supposing it to be collected on the surface of 

 the small piston, is 2 - 3 times the common logarithm of the number of times the 

 large cylinder is greater than the smaller one, multiplied by the force of the steam 

 in the boiler on a circular inch. Thus if the force be 120 inches of mercury, then 



