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principle of the bridge) for more deliberate
consideration.

Taking the designs in the list in
succession I come to No. 6, which not-
withstanding the calculable made by
the designer, accurate as far as they go, I
must also set aside as not sufficiently strong.
The designer assumes that the rule for cal-
culating the strength of a simple homoge-
neous beam is applicable to one built ac-
cording to this design in short lengths and
in several thicknesses, and therein is a se-
rious error at first starting, further he
has not taken into consideration the weight
of the beams themselves which should be
an element in the calculation; further he
takes the value of S (the ultimate trans-
verse strength) in my opinion at too high a
rate: doubtless the experiments made at
Sydney would give the value stated, but it
must be recollected that all such experi-
ments are made with carefully picked sea-
soned material of short lengths, small scan-
tling and straight grain under the most fa-
vorable circumstances, an allowance should
therefore be made for defect in strength of
less carefully chosen, less seasoned mate-
rial whose grain may be twisted and not
equal throughout the whole section of a
wide beam or plank. The arched form gi-
ven to the girders in this design would add
to their strength provided the abutments
were sufficiently substantial and immovea-
ble, and doubtless the designer felt he
wanted such addition, but as a simple in-
spection of the Sketch will show that the
portions of the abutments which of the de-
signer has availed himself are insufficient to
resist the great thrust of so flat a segment-
al arch, and as they may be moved by
earthquakes, they cannot be depended on to
compensate the deficiency of strength in the
beams. The designer's calculations of the
available strength as compared with the
possible weight to be thrown on the bridge
are very close, viz., 45 tons of capability
to meet a possible weight of 44 tons, and it
cannot be considered that the available
strength is taken too low at one fourth of
the calculated breaking weight, for it has
been ascertained by careful experiment that
any load beyond that destroys the elasticity
of the beam, and thus an element of des-
truction is at once introduced. It must be
further considered that a drove of cattle is
not a dead weight, but a moving weight
causing a vibration and resolving itself to
a certain extent into a percussive force,
which would add by so much to the load.
Taking all these points into consideration,
that a serious error has been made in the
calculations of the strength of the bridge,
which without that error are too close, com-
paring capability and duty, to be safely

carried into practice; that the ultimate
strength of the material is taken at too high
a rate; that no allowance is made for the
percussive force of a moving weight, nor for
the percussive force of high winds imping-
ing obliquely on the road way; I cannot
come to any other conclusion than that
stated at the beginning of my remarks on
this design. Besides that it is not an eco-
nomical design, the quantity of timber in
the beams alone, if judiciously disposed,
would have constructed the whole of a safe
and sufficient bridge, and the Estimate is
palpably too low. I have entered thus at
length into the consideration of this design
because the calculations at the commence-
ment of the description give a specious air
of sufficiency to it which might have im-
posed upon those who were not thoroughly
qualified by professional education to detect
their comparative fallacy, but in making
these remarks I do not wish it to be under-
stood that I am of opinion there was the
slightest intention on the part of the de-
signer to mislead.

Of No. 7 I will simply remark that the
tie rods on the whole strength of the
bridge depends are quite insufficient to meet
the tension that would be brought upon
them considering there would be a cer-
tain extent a transverse strain where the
struts rest upon the junction between the
double and single rods, besides that neither
the drawing nor the specification gives any
thing like sufficient information to enable
a builder to make a proper estimate of and
tender for the tender.

In No. 8, apparently by the same hand,
in consequence of the great obliquity of the
principals of the truss to the tiebeam, the
necessary great thrust and the smallness of
the abutting ends, which bear unrelieved
by any other abutments the whole thrust,
they would be unavoidably crippled; fur-
ther considering the great length of the
principals and the straining beam, and their
scantling, I believe there would be a ten-
dency to buckle which would be fatal to
the stability of the bridge. The design is
not an eligible one, and I cannot assign it
a favorable place in the competition.

Nos. 9, 10, and 11 are, I should judge,
by the same designer, and they require some
special consideration before they are placed
in the category of ineligible designs which
I regret is the place I must assign them.
No. 9 strikes an observer at first sight from
its simplicity, but on close examination I
cannot come to any other conclusion than
that it is very insufficient in strength.
With the depth of the girders I think no
reasonable fault can be found nor with the
sectional area of the sides of the girder, but
the sectional area of both the top and the