71. _Q._--Then, in computing the amount of metal necessary to give due
strength to a beam, the first point is to determine the velocity with which
the edge of the beam moves at that point were the strain is greatest?

_A._--The web of a cast-iron beam or girder serves merely to connect the
upper and lower edges or flanges rigidly together, so as to enable the
extending and compressing strains to be counteracted in an effectual manner
by the metal of those flanges. It is only necessary, therefore, to make the
flanges of sufficient strength to resist effectually the crushing and
tensile strains to which they are exposed, and to make the web of the beam
of sufficient strength to prevent a distortion of its shape from taking
place.

72. _Q._--Is the strain greater from being movable or intermittent than if
it was stationary?

_A._--Yes it is nearly twice as great from being movable. Engineers are in
the habit of making girders intended to sustain a stationary load, about
three times stronger than the breaking weight; but if the load be a movable
one, as is the case in the girders of railway bridges, they make the
strength equal to six times the breaking weight.

73. _Q._--Then the strain is increased by the suddenness with which it is
applied?

_A._--If a weight be placed on a long and slender beam propped up in the
middle, and the prop be suddenly withdrawn, so as to allow deflection to
take place, it is clear that the deflection must be greater than if the
load had been gradually applied. The momentum of the weight and also of the
beam itself falling through the space through which it has been deflected,