138 OF THE GENERATION AND [SECT. in. 



and down, vertically, in iron grooves, (see Plate in.) with as little friction as 

 possible. To its upper part is attached a chain, which passes over the two pulleys 

 n and n, through a tube in the bottom of the cistern head of the feed pipe, and 

 down the centre of the feed pipe C, to a hollow or bucket-shaped cast iron weight ; 

 the feed pipe being made of larger diameter in this part, when a self-regulating 

 damper is applied to a boiler, to admit the weight without blocking up the pipe 

 so as to prevent the descent of the feeding water. The weight is so adjusted by 

 filling it partly with lead, that it may just overcome the weight and friction of the 

 damper plate, chains, and pulleys, when there is no fire under the boiler ; con- 

 sequently, the damper plate will then be drawn up, and the chimney completely 

 open, at which time the weight will rest on the shoulders or projections at the 

 bottom of the feed pipe ; the chain being properly adjusted in length for that 

 purpose. Now as soon as a fire is applied so as to generate steam in the boiler, 

 the steam presses upon the surface of the water, and drives it up to a certain 

 height in the feed pipe ; and the weight, by becoming immersed in water, has part 

 of its gravitating force balanced, and therefore becomes no longer able to retain 

 the damper plate at its former height ; it will consequently descend till equilibrium 

 takes place, and partly closes the chimney, by which the draught of the fire will be 

 checked. Should it move so as to check it too much, less steam will be formed, 

 and the water will rise to a less height in the feed pipe, and part of the force of 

 the weight will be restored so as to raise the damper again : should the fire ever 

 become so fierce as to drive the water up into the cistern head, the weight 

 should be so far raised as nearly to shut the chimney ; when a damper shuts 

 perfectly close, there is a risk of inflammable air collecting and exploding in the 

 flues. 



A hand damper is, however, an appendage which a boiler should always have, 

 for when an engine is not in action, it will be useful partially to close it ; and no 

 boiler can be considered perfect which has not both a damper and the means of 

 entirely closing the aperture by which the air enters to supply the fire. 



258. Self-acting Air Regulator. The most direct method of governing the 

 action of a fire is to provide the passages which admit air with the means of 

 opening or closing them at pleasure ; and it is a still further advantage when this 

 is done by means of the force of the steam, so that as the steam increases beyond 

 its proper strength, it closes the aperture which admits air to the fire. A method 

 of constructing a self-acting regulator of this kind is shown in Plate i. It is 

 essential in applying it to make all other entrances to the burning fuel to shut as 

 perfectly close as possible. 



