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 together
different kinds of iron was not found to be favorable. The accidental
overheating of a boiler was found to reduce