hed in, just as would
happen to a spring to which an undue strain had been applied. This limit is
what is called the limit of elasticity; and whenever it is exceeded, the
bar, though it may not break immediately, will undergo a progressive
deterioration, and will break in the course of time. The limit of
elasticity of malleable iron when extended, or, in other words, the tensile
strain to which a bar of malleable iron an inch square may be subjected
without permanently deranging its structure, is usually taken at 17,800
lbs., or from that to 10 tons, depending on the quality of the iron. It has
also been found that malleable iron is extended about one ten-thousandth
part of its length for every ton of direct strain applied to it.

68. _Q._--What is the limit of elasticity of cast iron?

_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 t