es. The various strains, to which the materials of a bridge are
subjected--are compression, extension and detrusion.

Wood and Iron are the materials more generally employed in bridge
construction--and in this pamphlet we shall take the following as the
working strength of the materials--per square inch of section.

Tension. Compression. Detrusion.

Wood, 2000 1000 150

Wro't Iron, 15000 11000

Cast Iron, 4500 25000

=Tension.= If a weight of 2000 lbs. were hung to the lowest end of a
vertical beam, so that the line of action of the weight and axis of
the beam formed one and the same straight line--the tension on the
beam would be 2000 lbs. But, if the beam were inclined, and the force
acted in a vertical direction, then the strain would be increased in
the ratio of the increase of the diagonal of inclination over the
vertical;--suppose the beam is 20 ft. long and inclined at an angle of
45 deg.--and let 2000 lbs., as before, be suspended from its lower end.
Now the diagonal being 20 deg.,--the vertical will be 14.014 ft.--and the
strain will be found as follows,--

14.014 : 20 :: 2000 : 2854--lbs.

The greater the angle of inclination from the horizontal, the less the
strain from a given load--and when the beam is vertical the weight
causes the least strain.

=Compression.= If we load a vertical post with a weight of 2000 lbs.,
the strain of compression exerted upon the post will be 2000 lbs. Now,
if we incline the post--the strain will be increased, as we have shown
above under the head of tension, and in like manner, dependent upon
the inclination.

But when wood, iron, or any other material is used for a pillar or
strut, it has not only to resist a crushing force, but also a force
tending to bend or bulge it laterally.

A post of circular section with a length of 7 or 8 diameters will not
bulge with any force applied longitudinally, but will split. But if
the length exceeds this limit--it will be destroyed by an action
similar to that of a transverse strain.

A cast iron column of thirty diameters in length, is fractured by
bending; when the length is less than this ratio--by bending and
splitting off of wedge shaped pieces. But by casting the column
hollow, and swelling it in the middle, its strength is greatly
increased.

Barlow's formula for finding the weight that can be sustained by any
beam, acting as a pillar or strut, before bendin