122 OF THE GENERATION AND [SECT. m. 



cylinder to a succeeding time of opening them ; let the fraction of that interval 

 during which the steam valves are open be subtracted, and 30 times the 

 difference will be the space for steam in cubic feet in a low pressure engine. 



Thus, let it be a double acting engine where the steam is cut off at two-thirds 

 of the stroke ; then the whole stroke is the distance of the times of opening the 

 steam valves, and two-thirds is the fraction ; therefore 1 f is i, and 30 x $ is 

 10 cubic feet. 



215. In a high pressure boiler the same rule applies; only instead of being 

 the space in feet, 30 times the difference must be divided by the density of the 

 steam compared with the atmospheric steam as unity. 



This may be done with sufficient nearness in practice, by dividing by the number 

 of atmospheres equal to the force of the steam in the boiler. 



If in a double acting high pressure engine, which admits the steam only during 

 half the stroke, the force of the steam in the boiler be 4 atmospheres, then for each 

 cubic foot of water the boiler is to boil off per hour there should be 



( ~ $> = 3-8 cubic feet of space for steam. 



216. Even in a double engine, which is intended to act at full pressure 

 throughout the stroke, there is the time of opening and closing the valves to be 

 deducted ; and in some of the usual modes one-fourth of the stroke at least is 

 expended, so that we can scarcely in any case say that less than 8, divided by the 

 atmospheres representing the force of the steam in the boiler, should be allowed as 

 the space in feet for steam for each cubic foot of water boiled off" per hour. 



217. Space for Water in a Boiler. That there should be water to cover the 

 sides of the boiler a little higher than the flues, is clear ; but there is another con- 

 dition which is less obvious but of considerable importance in effect, and it is 

 particularly interesting in steam boats, where we wish to have neither more of 

 space nor weight than is absolutely necessary. 



The quantity of water an engine consumes is not admitted with perfect re- 

 gularity; it is most equably done when forced in by a pump worked by the 

 engine, and the portion admitted regulated by a float ball. See Plate i. Fig. 2. 



The quantity necessary to produce the steam must however be admitted, and its 

 temperature we will suppose to be 100; now the water in the boiler we will 

 suppose to be 225, and the proportion of the quantity in the boiler to that admitted 

 ought to be such that the temperature should not be lowered, so as to reduce the 

 force of the steam one-thirtieth part ; otherwise a manifest disadvantage must take 

 place in the action of the steam. But the depression of the temperature of the 

 water 2 will diminish its elastic force one-thirtieth ; hence, supposing the quantity 



