SECT, ix.] OF STEAM ENGINES. 281 



OF IMPELLING MACHINERY FOR MANUFACTURING PURPOSES. 



583. IRON MANUFACTURE. In this manufacture the steam engine is applied 

 to blowing machines, forge hammers, rolling, flatting, and slitting machines, and 

 various other purposes. 



584. BLOWING MACHINES. The object of this machine is to supply oxygen 

 to furnaces, either for melting or reducing ores to the metallic state ; hence, in 

 order that the effect may be the same, or nearly so, when the same fuel is used, 

 the supply of oxygen should be the same. But in the same bulk of dry air there 

 is nearly 10 per cent less oxygen at 85 than at 32; and 12 per cent less 

 when the air at 85 is saturated with vapour ; consequently, if 1 500 feet per 

 minute be a sufficient supply for a furnace in winter, it may require 1625 feet per 

 minute in summer, to have the same effect ; and the difference ought clearly to 

 be gained partly by the aperture being enlarged, and partly by increasing the 

 intensity of the blast. 



The blast is usually produced by condensing the air, till it will sustain a column 

 of from four to six inches and a half of mercury, (one and a half to two Ibs. 

 per circular inch,) according to the quality of the coal ; and the mean between 

 these is most generally found to answer : the quantity discharged varies from 

 3000 to 1200 feet per minute. 



If v be the velocity of the piston of a blowing cylinder in feet per minute, p the 

 force of compression in Ibs. per circular inch, and a the diameter of the blowing 

 cylinder in inches, then, allowing that the friction increases the power from 1 to 

 1'25, we have 1'25 p v cf = the power in Ibs. raised one foot high per minute, 

 when the stroke is effective in both directions, and half that when in one direction 

 only. l The capacity of the air chest should be proportioned by the principle given 

 in art. 211, and the passages to it should be about one-twentieth of the area of the 

 cylinder. The quantity of air delivered into the chest will be about one-fifth less 

 than the capacity of the cylinder, when taken at atmospheric density, partly 

 through escape by the valves, and by the air not entering till the space within the 

 cylinder is rarefied so as to produce the velocity. 



For this as well as all other parts of iron manufacture, the double acting con- 

 densing engine, prepared to work either expansively or at full power, will be 

 found the best. (See art. 421.) 



1 The rule is only an approximation, but nearly correct for small degrees of compression ; in 

 greater ones the principles of the note to art. 377 should be applied. 



2 N 



