SECT, in.] CONDENSATION OF STEAM. 123 



introduced at each time to be 1, and the quantity in the boiler to be #, we must 

 have 



(1 x 100) + (x x 225) = 225 - 2 = 223- 



l+x 



whence we find x = 62 nearly ; that is, there must be 62 times as much water in 

 the boiler as is introduced at one feed, otherwise the force of the steam will be 

 lowered more than one-thirtieth. The rule applies to both high and low pressure 

 steam ; for the variation by a change of 2 of temperature is nearly proportional. 

 The more frequently the feeding apparatus acts, the less water we require, and we 

 also see a stronger motive for using hot water for the boiler than that of barely 

 saving fuel ; as the colder it is, the more the steam will be reduced. If a boiler be 

 fed at every stroke, it should have 5 cubic feet of water for each cubic foot of 

 steam it is capable of boiling off per hour, whether the boiler be high or low 

 pressure. 



218. The self-acting feeding apparatus must be delicately adjusted to reduce its 

 intervals to even twice that time, and therefore such boilers require at least 10 cubic 

 feet of water for each cubic foot of water boiled off per hour. But a mode of 

 rendering the self-acting feed regular is shown in Plates i. and n. 



219. It is shown therefore, that to limit a low pressure steam boiler of a double 

 acting engine, with a self-acting feed, to a change of elastic force not exceeding 

 one-thirtieth, we must have 10 feet space for steam and 10 for water for each 

 cubic foot of water the boiler commonly generates in an hour, or for each horse 

 power ; and that if the steam be cut off before the stroke is completed, a greater 

 space must be allowed for steam. 



220. It is usually stated that there should be 25 cubic feet of boiler for each 

 horse power, others say 20 is sufficient, and even so low as 8 has been proposed ; 

 while another party states that there is no relation between the cubic contents of the 

 boiler and the power. We have now however shown, on unquestionable principles, 

 what ought to determine the least contents of the boiler ; and it appears that to 

 omit the estimation either of the surface to receive heat or the capacity, would 

 be erroneous. Both should be considered and determined from the circumstances 

 of the case. 



s 



OF THE POWER OF Low PRESSURE BOILERS. 



221. The power of boilers to produce steam is considerably affected by the 

 loss of heat, and a small boiler more so than a large one. 



It is one of those cases which seems to be incapable of being investigated 

 otherwise than by experience. In a boiler proportioned to the effect to be pro- 



