AMERICAN INSTITUTE. 577 



nished with all the safety apparatus ever invented; and with 

 them, a properly proportioned and well fitted safety valve and 

 proper gauge cocks are all that are needed to secure the greatest 

 degree of safety which the use of a steam boiler admits ; though 

 convenience and economy in working will recjuire the addition of 

 a correct steam gauge, this will add nothing to the safety, 



R. L. Pell remarked — Since the days of Watt there has been 

 no very important improvement made in the steam engine, 

 though a multitude of forms have been introduced. Sundry 

 metals have been proposed for the construction of boilers, among 

 them copper, the advantages of which over iron are not sufficiently 

 great to warrant the diiference in price. 



Iron, then, may be considered the best material for that pur- 

 pose, sheet iron a quarter of an inch thick, is generally sufficient 

 for a high tension of steam, say from 125 to 150 pounds to the 

 square inch. I would not use thinner iron than this, as it would 

 be apt to leak and cause oxidation, and consequent explosions, 

 which almost invariably occur at the instant the engine starts, 

 caused by the instantaneous generation of steam by the sudden 

 motion given to the water. Thin iron plates cannot be rivetted 

 as well as thicker ones, and generally speaking the rivets are made 

 too small, and consequently the boiler plates are not as strongly 

 put together as they should be. If the iron made use of for 

 boilers is good in quality, no change can be produced by wear; it 

 will continue good to the last. It has been proposed as a pre- 

 ventive to boiler explosions; that a pipe leading from a cistern 

 of cold water, should pass through the boiler, whilst the stop cock 

 that opens the passage is kept closed by a chain within the boiler, 

 and in which chain one link is made of fusible metal, capable of 

 being fused at that temperature above which the boiler is exposed 

 to the danger of explosion. 



An alloy, composed of one part lead, three tin, and five of bis- 

 mutli, will fuse at the ordinary temperature of boiling water; and 

 alloys of the same metals, in different proportions, will fuse at 

 temperatures from 300 to 400 degrees. When we extinguish the 

 fires under a boiler, the steam will immediately condense, and 

 form a vacuum, and were it not for the safety valve opening 

 inward, and balanced by a weight to keep it closed until relieved 

 from the pressure of the pent up steam, the atmosphere would 

 have a strong tendency to crush it inward. Engineers now ex- 

 press an opinion adverse to the use of fusible safety plugs, and 

 they are consequently generally abandoned. 



