e damage was caused by 10,000 deflections, each deflection being
such as was due to half the load that produced a large permanent
deflection.

78. _Q._--The power of a rod or pillar to resist compression becomes very
little when the diameter is small and the length great?

_A._--The power of a rod or pillar to resist compression, varies nearly as
the fourth power of the diameter divided by the square of the length. In
the case of hollow cylindrical columns of cast iron, it has been found,
experimentally, that the 3.55th power of the internal diameter, subtracted
from the 3.55th power of the external diameter, and divided by the 1.7th
power of the length, will represent the strength very nearly. In the case
of hollow cylindrical columns of malleable iron, experiment shows that the
3.59th power of the internal diameter, subtracted from the 3.59th power of
the external diameter, and divided by the square of the length, gives a
proper expression for the strength; but this rule only holds where the
strain does not exceed 8 or 9 tons on the square inch of section. Beyond 12
or 13 tons per square inch of section, the metal cannot be depended upon to
withstand the strain, though hollow pillars will sometimes bear 15 or 16
tons per square inch of section.

79. _Q._--Does not the thickness of the metal of the pillars or tubes
affect the question?

_A._--It manifestly does; for a tube of very thin metal, such as gold leaf
or tin foil, would not stand on end at all, being crushed down by its own
weight. It is found, experimentally, that in malleable iron tubes of the
respective thicknesses of .525, .272, and .124 inches, the resistances per
square inch of section are 19.17, 14.47, and 7.47 tons respectively. The
power of plates to resist compression varies nearly as the cube, or more
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,