mass relatively with the magnitude of the deflecting pressure, and the
rapidity with which that pressure is applied and removed. Thus if a force
or weight be very suddenly applied to the middle of a ponderous beam, and
be as suddenly withdrawn, the inertia of the beam will, as in the case of
the collision of bodies, tend to resist the force, and thus obviate
deflection to a considerable extent; but if the pressure be so long
continued as to produce the amount of deflection due to the pressure, the
effect of the inertia in that case will be to increase the deflection.

76. _Q._--Will the pressure given to the beam of an engine in different
directions facilitate its fracture?

_A._--Iron beams bent alternately in opposite directions, or alternately
deflected and released, will be broken in the course of time with a much
less strain than is necessary to produce immediate fracture. It has been
found, experimentally, that a cast-iron bar, deflected by a revolving cam
to only half the extent due to its breaking weight, will in no case
withstand 900 successive deflections; but, if bent by the cam to only one
third of its ultimate deflection, it will withstand 100,000 deflections
without visible injury. Looking, however, to the jolts and vibrations to
which engines are subject, and the sudden strains sometimes thrown upon
them, either from water getting into the cylinder or otherwise, it does not
appear that a strength answering to six times the breaking weight will give
sufficient margin for safety in the case of cast-iron beams.

77. _Q._--Does the same law hold in the case of the deflection of malleable
iron bars?

_A._--In the case of malleable iron bars it has been found that no very
perceptible damage was caused by 10,000 deflections, each deflection being