SECT, vi.] CONDENSING ENGINES. 199 



246 - 6 

 At the full pressure, the fuel was 246'6 Ibs. ; in this case it is -^- =117 Ibs. 1 



117 

 hence -jg- = 6'5 Ibs. per horse power ; the advantage is therefore as 6'5 : 9'2, or 



as 10 : 14. 2 For small engines this quantity requires to be increased in the ratio 

 given in the table, art. 221. 



423. The mode of cutting off the steam by giving two movements to the slide 

 during the stroke is shown in Plate v. : Fig. 2. shows the position of the slide 

 when the piston is descending and the steam cut off, with the passage D to the 

 condenser still open. Slides have the defect of requiring a separate passage to 

 introduce the steam to expel the air from the engine at the time of starting, techni- 

 cally called " blowing through ;" but in other respects they seem to afford the 

 most simple and durable means of opening and closing the passages. 



COMBINED CYLINDER ENGINES. 



424. In Hornblower's engine with two cylinders the steam acts at full pressure 

 in the one, and expansively in the other : as a single engine it is decidedly inferior 

 to Boulton and Watt's construction in every respect, except that of the moving 

 force being more nearly uniform, for there is the additional friction of the small 

 piston ; and it is a singular fact, that a single engine of this kind is more complex 

 than a double one. As mine engines they appear to be nearly abandoned, and 

 therefore it is not necessary to occupy space in describing a species which will be 

 sufficiently understood by imagining two single engines acting on one beam, the 

 one of which works at full pressure, and the steam which propels it acts expan- 

 sively in the other cylinder during the next stroke. In both cylinders the steam 

 has to change from the upper to the lower sides of the piston during the ascent. 

 The ratio of the size of the expansion cylinder to the other should be determined 

 by the same rule as for double engines of this kind (art. 426), and in other 

 respects the proportions should be as for single engines. 



425. The double engine with combined cylinders. This engine will be under- 

 stood most easily with a simple mode of letting on and off the steam. Let C be 

 the small cylinder, Plate vi. Fig. 3. and D the large one, and S the place where 

 the steam enters the pipes. The steam enters the small cylinder at a when the 

 piston descends, and the portion below its piston passes through b, and rising in the 



1 This is the same as raising 27,000,000 Ibs. 1 foot high by 1 bushel of coals. 



2 If we take the mean between 6-5 and 9-2 or 7-85, it is what we may expect to be the ordinary 

 consumption of an engine with a variable resistance, when of the best kind. 



