and indeed it is generally less than this. The ultimate tensile strength,
therefore, of malleable iron is four times greater than that of cast iron,
but the crushing strength of cast iron is between three and four times
greater than that of wrought iron. It may be stated, in round numbers, that
the tensile strength of malleable iron is twice greater than its crushing
strength; or, in other words, that it will take twice the strain to break a
bar of malleable iron by drawing it asunder endways, than will cripple it
by forcing it together endways like a pillar; whereas a bar of cast iron
will be drawn asunder with one sixth of the force that will be required to
break or cripple it when forced together endways like a pillar.

65. _Q._--What is the cohesive strength of steel?

_A._--The ultimate tensile strength of good cast or blistered steel is
about twice as great as that of wrought iron, being about 130,000 lbs. per
square inch of section. The tensile strength of gun metal, such as is used
in engines, is about 36,000 lbs. per square inch of section; of wrought
copper about 33,000 lbs.; and of cast copper about 19,000 lbs. per square
Inch of section.

66. _Q._--Is the crushing strength of steel greater or less than its
tensile strength?

_A._--It is about twice greater. A good steel punch will punch through a
plate of wrought iron of a thickness equal to the diameter of the punch. A
punch therefore of an inch diameter will pierce a plate an inch thick. Now
it is well known, that the strain required to punch a piece of metal out of
a plate, is just the same as that required to tear asunder a bar of iron of
the same area of cross section as the area of the surface cut. The area of
the surface cut in this case will be the circumference of the punch, 3.1416