Polytechnic Association Proceedings. 711 



whole is protected in such a manner that the wood work will not 

 be overheated. After the fire is started a single pailful! of nut 

 coal is sufficient to keep the car comfortably warm in the coldest 

 day. The heater has been successfully applied to cars at the 

 North. 



Mr. Julius Silversmith spoke on mining metallurgy. 



poors SECTIONAL BOILER. 



Mr. John B. Root illustrated by diagrams on the blackboard his 

 wi'ought iron sectional safetj'^ boiler, which is fully explained in the 

 following paper, furnished by him: 



The essential features to which this boiler owes its entire safety 

 from explosion, are the small diameters of its parts and the sub- 

 division of the water and steam in small compartments. It needs 

 no argument to show the superiority, in point of strength, of small 

 over large diameters; the tubes used are four inches in diameter 

 and one-eighth of an inch in thickness; they are as strong as twelve 

 inches in diameter and three-eighths of an inch thick, or twenty-four 

 inches diameter and three-fourths of an inch thick, or thirtj'-six inches 

 diameter and one and one-eighth inch thick, or forty-eight inches 

 diameter and one and one-half inch thick, and so on in proportion 

 to diameter. There is a further advantage in favor of the strengfth 

 of tubes, such as used in the safety boiler; they have no riveted 

 joints; while boilers of large diameters are always made of sheets 

 and riveted, which diminishes their strength from thirty to forty- 

 four per cent, as compared with a solid cylinder. The above 

 extreme thicknesses are never seen in practice, but the diameters 

 are common. 



These tubes are made of iron that will bear 45,000 pounds per 

 square inch tensile strain, across the sheets. They are all tested at 

 300 pounds pressure per square inch, and are good for two or three 

 times this pressure. 



Their immense strength to bear pressure is principiJly owing to 

 their small diameter; but also to the use of the best chai'coal iron 

 in their manufacture. 



The circumference of a boiler shell of four feet diameter is 150 

 inches; hence if the pressure usually carried is fifty pounds per 

 square inch (many carry 100 to 150 pounds), there is constantly 

 being exerted upon each inch in length of the shell a bursting 

 pressure of 7,500 pounds, and if the boiler is fifteen feet or 180 

 inches long, the entire pressure, which is tending to rend the boiler 



