[Illustration: BRIDGE OVER THE BLAAUW KRANTZ RAVINE, CAPE COLONY.]

In designing the structure the following points had to be considered: (1)
That, on account of the great height above the ground, and on account of
the high price of timber at the site, the structure could be easily
erected without the use of scaffolding supporting it as a whole. (2) That,
on account of the high freights to Port Alfred, the quantity of iron in
the structure should be as small as possible. (3) That the single parts of
the principal span should be easy to lift, and that there should be as few
of them as possible. For this latter reason most of them were made in
lengths of 20 ft. and more. The question of economy of material presented
itself as a comparison between a few standard types, viz., the girder
bridge of small independent spans; the cantilever bridge, or the
continuous girder bridge in three large spans; the single girder bridge
with one large span and several small spans; and the arch with small
girder spans on each side. The suspension bridge was left out of question
as inadmissible. A girder bridge with small independent spans on rocker
piers would probably have been the most economical, even taking into
account the great height of the piers near the middle of the ravine, but
there would have been some difficulty in holding those piers in position
until they could be secured to the girders at the top; and, moreover, such
a structure would have been strikingly out of harmony with the character
of the site. On the other hand, a cantilever or continuous girder bridge
in three spans--although such structures have been erected in similar
localities--could not enter into comparison of simple economy of material,
because such a design would entirely disregard the anomaly that the
greater part of the structure, viz., the side spans, being necessarily
constructed to carry across a large space, would be too near the ground to
justify the omission of further supports. The question was, therefore,