than that at the circumference of the driving wheel, in the proportion of their slower motion, or in the proportion of the circumference of the driving wheel to the length of a double stroke of the engine.  If the diameter of the driving wheel be 5-1/2 feet, its circumference will be 17.278 feet, and if the length of the stroke be 18 inches, the length of a double stroke will be 3 feet.  The pressure on the pistons must therefore be greater than the traction at the circumference of the driving wheel, in the proportion of 17.278 to 3, or, in other words, the mean pressure on the pistons must be 17,278 lbs.; and the area of cylinders, and pressure of steam, must be such as to produce conjointly this total pressure.  It thus becomes easy to tell the volume and pressure of steam required, which steam in its turn represents its equivalent of water which is to be evaporated from the boiler, and the boiler must be so proportioned, by the rules already given, as to evaporate this water freely.  In the case of a steam vessel, the mode of procedure is the same, and when the resistance and speed are known, it is easy to tell the equivalent value of steam.

STRENGTH OF BOILERS.

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