THE STEAM-ENGINE. 



505 



portion of its heat preparatory to being once more put in circulation through 

 the machine. 



The entire quantity of hot water pumped into the cistern C, is not always 

 necessary for the boiler. A waste-pipe may be provided for carrying off the 

 surplus, which may be turned to any purpose for which it may be required ; or 

 it may be discharged into a cistern to cool, preparatory to being restored to 

 the cold cistern, in case water for the supply of that cistern be not sufficiently 

 abundant. 



Another method of arranging a self-regulating feeder is shown in fig. 59. 

 A is a hollow ball of metal attached to the end of a lever, whose fulcrum is at 

 B. The other arm of the lever C is connected with the stem of a spindle- 

 valve, communicating with a tube which receives water from the feeding- 

 cistern. Thus, when the level of the water in the boiler subsides, the ball A 

 preponderating over the weight of the opposite arm, the lever falls, the arm C 

 rises and opens the valve, and admits the feeding-water. This apparatus will 

 evidently act in the same manner and on the same principle as that already 

 described. 



The mouth of the tube by which the feed is introduced should be placed at 

 that part of the boiler which is nearest the end of the flues which issue into 

 the chimney. By such means the temperature of the water in contact with 

 those Hues will be lowest at the place where the temperature of the heated air 

 intended to act upon it is also lowest. The difference of the temperatures 

 will therefore be greater than it would be if the point of the boiler contain- 

 ing water of a higher temperature was left in contact with this part of the flue. 



It is necessary to have a ready method of ascertaining at all times the pres- 

 sure of the steam which is used in working the engine. For this purpose a 

 benl tube containing mercury is inserted into some part of the apparatus, which 

 has free communication with the steam. Let ABC, fig, 60, be such a tube. 



Fig. 60. 



The pressure of the steam forces the mercury down in the leg A B, and up in 

 the leg B C. If the mercury in both legs be at exactly the same level, the 

 pressure of the steam must be exactly equal to that of the atmosphere , because \ 



