462. I shall commence the description of the principle of this bridge
by performing some experiments upon a tube, which I hold in my hand.
The tube is square, 1" × 1" in section, and 38" long. It is made of
“tin,” and weighs rather less than a pound.
463. Here is a solid rod of iron of the same length as the tube, but
containing considerably more metal. This is easily verified by weighing
the tube and the rod one against the other. I shall regard them as two
girders, and experiment upon their strength, and we shall find that,
though the tube contains less substance than the rod, it is much the
stronger.
464. I place the rod on a pair of supports about 3' apart; I then
attach the tray to the middle of the rod: 14 lbs. produce a deflection
of 0"·51, and 42 lbs. bends down the rod through 3"·18. This is a
large deflection; and when I remove the load, the rod only returns
through 1"·78, thus showing that a permanent deflection of 1"·40 is
produced. This proves that the rod is greatly injured, and demonstrates
its unsuitability for a girder.
465. Next we place the tube upon the same supports, and treat it in the
same manner. A load of 56 lbs. only produces a deflection of 0"·09,
and, when this load is removed, the tube returns to its original
position: this is shown by the cathetometer, for a cross is marked on
the tube, and I bring the image of it on the horizontal wire of the
telescope before the load of 56 lbs. is placed in the tray. When the
load is removed, I see that the cross returns exactly to where it was
before, thus proving that the elasticity of the tube is unimpaired. We
double the load, thus placing 1 cwt. in the tray, the deflection only
reaches 0"·26, and, when the load is removed, the tube is found to be
permanently deflected by a quantity, at all events not greater than
0"·004; hence we learn that the tube bears easily and without injury
a load more than twice as great as that which practically destroyed a
rod of wrought iron, containing more iron than the tube. We load the
tube still further by placing additional weights in the tray, and with
140 lbs. the tube breaks; the fracture has occurred at a joint which
was soldered, and the real breaking strength of the tube, had it been
continuous, is doubtless far greater. Enough, however, has been borne
to show the increase of strength obtained by the tubular form.
466. We can explain the reason of this remarkable result by means of
Fig. 64. Were the thin portion of the girder E F made of two parts
placed side by side, the strength would not be altered. If we then
imagine the flange A B widened to the width of C D, and the two parts
which form E F opened out so as to form a tube, the strength of the
girder is still retained in its modified form.
467. A tube of rectangular section has the advantage of greater depth
than a solid rod of the same weight; and if the bottom of the tube be
strong enough to resist the extension, and the top strong enough to
resist the compression, the girder will be stiff and strong.
468. In the Menai Tubular Bridge, where a gigantic tube supported at
each end bridges over a span of four hundred and sixty feet, special
arrangements have been made for strengthening the top. It is formed of
cells, as wrought iron disposed in this way is especially adapted for
resisting compression.
469. We have only spoken of rectangular tubes, but it is equally true
for tubes of circular or other sections that when suitably constructed
they are stronger than the same quantity of material, if made into a
solid rod.
470. We find this principle in nature; bones and quills are often found
to be hollow in order to combine lightness with strength, and the
stalks of wheat and other plants are tubular for the same reason.