feet away. Then comes a leap of one thousand seven
hundred feet to the four on the island of Inchgarvie, followed by a
similar bound to the four near the northern bank, and then a half-stride
again of six hundred and eighty feet to land.

The three sets of caissons once being in their places, and the stone
piers built on top of them, people at last began to see the beginning of
the Forth Bridge. From each of the four piers in each group there slowly
rose a huge steel tubular column, twelve feet in diameter, each pair
leaning inwards, so that though at their bottoms they stood one hundred
and twenty feet away from the pair on the opposite side (that being the
width of the base of the bridge), the head of both pairs were only
separated by a distance of thirty-three feet. This was done to afford
greater resistance to the wind. Each group of four columns forms what
are called the towers, and rises to a height of three hundred and thirty
feet. They are firmly braced together by tie-girders and cross tubes
nearly as large as themselves. They were erected section by section,
rivets and hammers being used instead of trowel and mortar. Scarcely
were their summits united when, from their feet, there began to spring
on either side the great tubes forming the lower part of the arch. In
the cantilever construction, the bridge grows right and left from its
piers at the same moment, because balance must be maintained. As the
lower arched tubes just mentioned stretched further over the water,
sloping girders started downward from the tower top to meet them, and
they were soon connected by lighter cross-ties. Tubes were used for the
arch because they are best suited to bear the _compression_ strain
caused by a train passing over the bridge. The girder form was chosen to
stretch downward from the tower top because it is better able to bear
the tension or _pulling_ strain. They together form what is called a
cantilever; if you lay the letter V on its side, the open end will
represent roughly the place where the arch and girders start from the
tower. Thus we see how the two strengths of suspension--cable and arch
are combined in the Forth Bridge.

When the sets of cantilevers from the grouped piers had grown out toward
one another till they were separated by only three hundred and fifty
feet, the gap was spanned by a connecting girder, the joints between it
and the cantilever being sufficiently loose to allow of the expansion
and contrac