298. Q.—What strain should the iron of boilers be subjected to in working?
A.—The iron of boilers, like the iron of machines or structures, is capable of withstanding a tensile strain of from 50,000 to 60,000 lbs. upon every square inch of section; but it will only bear a third of this strain without permanent derangement of structure, and it does not appear expedient in any boiler to let the strain exceed 4,000 lbs. upon the square inch of sectional area of metal, especially if it is liable to be weakened by corrosion.
299._Q._—Have any experiments been made to determine the strength of boilers?
A.—The question of the strength of boilers was investigated very elaborately a few years ago by a committee of the Franklin Institute, in America, and it was found that the tenacity of boiler plate increased with the temperature up to 550 deg., at which point the tenacity began to diminish. At 32 deg., the cohesive force of a square inch of section was 56,000 lbs.; at 570 deg., it was 66,500 lbs.; at 720 deg., 55,000 lbs.; at 1,050 deg., 32,000 lbs.; at 1,240 deg., 22,000 lbs.; and at 1,317 deg., 9,000 lbs. Copper follows a different law, and appears to be diminished in strength by every addition to the temperature. At 32 deg. the cohesion of copper was found to be 32,800 lbs. per square inch of section, which exceeds the cohesive force at any higher temperature, and the square of the diminution of strength seems to keep pace with the cube of the increased temperature. Strips of iron cut in the direction of the fibre were found to be about 6 per cent. stronger than when cut across the grain. Repeated piling and welding was found to increase the tenacity of the iron, but the result of welding