SECT, ii.] PROPERTIES OF STEAM. 103 



For coal smoke s = 1'05, and the formula 



B (459 + 



a = 



2825 V h (f 78) ' 

 For low pressure steam 



- = , the area in square inches. 

 56 / h 



The application of the formula, with some simple rules suggested by it for 

 engines of different powers, will be given with the proportions of fire-places : the 

 investigation is given here to separate in some measure scientific inquiry from 

 practical details. 



OF THE ESCAPE OF STEAM AT SAFETY VALVES. 



173. This is a subject which has been little studied. If we suppose the steam 

 to be of the same density as atmospheric air, its elastic force is nearly twice as 

 great, and it would rush into the atmosphere with the same velocity that atmo- 

 spheric air rushes into a vacuum. Also, in any case, whether the elastic force of 

 the steam be greater or less than this, if n be the number of times its specific 

 gravity exceeds that of atmospheric steam, when that of air at the same pressure 

 is 1, we shall have v = S */ (n 1) A, h being the height of a uniform atmosphere 

 of air. A uniform atmosphere equivalent to thirty inches of mercury is 28,000 

 feet; hence, 1340 */ n 1 =v in feet per second. 



174. In certain cases this will be aided by the buoyancy of the escaping 

 steam, and in very dense steam it may be slightly retarded by the same cause ; but 

 these effects are not so great as to need to be introduced in the calculation. They 

 may, however, be sensibly observed by turning the aperture up or down ; a light 

 fluid escaping with the greatest velocity when the aperture is turned up, a heavy 

 one when it is downwards. 



175. Let a be the area of the aperture in square inches ; then, reducing the 

 velocity for the contraction in passing the aperture, we have 300 a *J n 1 = the 

 quantity of steam escaping in cubic feet per minute ; n being the number of times 

 the density is greater than that of atmospheric steam : or making c = the number 

 of cubic feet of steam generated in a second, then 



= a, the area in inches. 



5n V 1 



This quantity should obviously be the greatest which the fire could, under any 

 possible circumstances, produce. 



When n is less than 1, or the density less than the density of atmospheric steam, 



