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
inches, multiplied by the thickness of the plate, 1 inch, which makes the
area of the cut surface 3.1416 square inches. The area of the point of the
punch subjected to the pressure is .7854 square inches, so that the area
cut to the area crushed is as four to one. In other words, it will require
four times the strain to crush steel that is required to tear asunder
malleable iron, or it will take about twice the strain to crush steel that
it will require to break it by extension.

67. _Q._--What strain may be applied to malleable iron in practice?

_A._--A bar of wrought iron to which a tensile or compressing strain is
applied, is elongated or contracted like a very stiff spiral spring, nearly
in the proportion of the amount of strain applied up to the limit at which
the strength begins to give way, and within this limit it will recover its
original dimensions when the strain is removed. If, however, the strain be
carried beyond this limit, the bar will not recover its original
dimensions, but will be permanently pulled out or pus