_A._--It is commonly taken at 15,300 lbs. per square inch of section; but
this is certainly much too high, as it exceeds the tensile strength of
irons of medium quality. A bar of cast iron if compressed by weights will
be contracted in length twice as much as a bar of malleable iron under
similar circumstances; but malleable iron, when subjected to a greater
strain than 12 tons per square inch of section, gradually crumples up by
the mere continuance of the weight. A cast-iron bar one inch square and ten
feet long, is shortened about one tenth of an inch by a compressing force
of 10,000 lbs., whereas a malleable iron bar of the same dimensions would
require to shorten it equally a compressing force of 20,000 lbs. As the
load, however, approaches 12 tons, the compressions become nearly equal,
and above that point the rate of the compression of the malleable iron
rapidly increases. A bar of cast iron, when at its breaking point by the
application of a tensile strain, is stretched about one six-hundredth part
of its length; and an equal strain employed to compress it, would shorten
it about one eight-hundredth part of its length.

69. _Q._--But to what strain may the iron used in the construction of
engines be safely subjected?

_A._--The most of the working parts of modern engines are made of malleable
iron, and the utmost strain to which wrought iron should be subjected in
machinery is 4000 lbs. per square inch of section. Cast iron should not be
subjected to more than half of this. In locomotive boilers the strain of
4000 lbs. per square inch of section is sometimes exceeded by nearly one
half; but such an excess of strain approaches the limits of danger.

70. _Q._--Will you explain in what way the various strains subsisting in a
steam engine may be resolved into tensile and crushing strains; also in
what way the magnitude of those strains may be determined?