nearly as the 2.878th power of their thickness; but this law only holds so
long as the pressure applied does not exceed from 9 to 12 tons per square
inch of section. When the pressure is greater than this the metal is
crushed, and a new law supervenes, according to which it is necessary to
employ plates of twice or three times the thickness, to obtain twice the
resisting power.

80. _Q._--In a riveted tube, will the riveting be much, damaged by heavy
strains?

_A._--It will be most affected by percussion. Long-continued impact on the
side of a tube, producing a deflection of only one fifth of that which
would be required to injure it by pressure, is found to be destructive of
the riveting; but in large riveted structures, such as a ship or a railway
bridge, the inertia of the mass will, by resisting the effect of impact,
prevent any injurious action from this cause from taking place.

81. _Q._--Will the power of iron to resist shocks be in all cases
proportional to its power to resist strains?

_A._--By no means. Some cast iron is very hard and brittle; and although it
will in this state resist compression very strongly, it, will be easily
broken by a blow. Iron which has been remelted many times generally falls
into this category, as it will also do if run into very small castings. It
has been found, by experiment, that iron of which the crushing weight per
square inch is about 42 tons, will, if remelted twelve times, bear a
crushing weight of 70 tons, and if remelted eighteen times it will bear a
crushing weight of 83 tons; but taking its power to resist impact in its
first state at 706, this power will be raised at the twelfth remelting to
1153, and will be sunk at the eighteenth remelting to 149.