The Sun.
London, England, 1850-04-25 00:00:00

INSTITUTION OF CIVIL ENGINEERS.

On Tuesday, April 23, (William Cubitt, Esq , Pre-
sident, in the chair,) the paper read was a " Descrip-
tion of the Insistent Pontoon Bridge, at the Dublin
Terminus of the Midland Great Western Railway of
Ireland," by Mr. R. Mallet, M. Inst. C. E.
This bridge was stated to be situated on the line of
approach from the city to the terminus, and formed
a passage over one branch of the Royal Canal, where
it crossed the Phibsborough-road, upon the Foster
Aqueduct. By the Act it was provided that the na•
vigation of the canal should be as free and unimpeded
as possible ; and from the circumstance of there
being only a height of 16 inches between the intended
surface of the road and that of the water of the canal,
it necessarily involved the placing of some kind of
moveable bridge, of rather peculiar construction.
After due consideration, the one described in the
paper was designed and adopted, as being more suit-
able to the peculiarities of the situation than any
other, owing to the water channel being only 17 feet
4 inches in width, and that the passage to be made
across it required to be at least 50 feet in breadth.
The general idea of this form of moveable bridge
was that of a pontoon, or flat-bottomed boat, con-
structed of iron, the breadth being nearly equal to
that of the water space to be crossed, and the length
about equal to the width of roadway required. The
deck beams of this pontoon projected over the sides,
and rested, while in situ, upon a rabbate, or continu-
ous recess, formed along the top course of each quay
wall, but while the pontoon was floating light, the
' projecting deck beams were two inches clear of this
rabbate, and the roadway platform constituting the
deck of the pontoon was elevated to an equal height
above the level of the top of the quay walls, or land,
on each side ; in this state the pontoon could be freely
and readily pushed along the canal for a distance of
rather more than its own length, until it was brought
opposite to a lye-by, provided by increasing the width
of the canal at this point, and being put therein, the
navigation was perfectly free.
As a pontoon afloat would form a very unstable
roadway for carriages, means were provided for allow-
ing it to settle down in the water, and rest firmly
upon the rabbates; and also for again rais'ng it
rapidly, so as to float clear of the rabbates, and enable
it to be moved away into the lye-by. For this pur-
pose two large valves were placed in the bottom of
the pontoon, one near each end, by which water was
allowed to enter, and sink the pontoon, until it hung
upon the projecting deck-beams. For removing this
water, when it was required to float the pontoon, a
large syphon, of a particular construction, was pro-
vided, which was capable of being brought instantly
into use, and of being as quickly detached when a
sufficiency of water had been withdrawn to enable
the pontoon to be moved. These operations were
stated to be performed very readily by one man, the
navigation being cleared in four minutes, and the
roadway restored in less than three minutes.
The details of the construction c f the pontoon, of
the sy phon, and all other parts of the work were
then minutely given; also the total cost of the struc-
ture, whirl, exclusive of the mascnry, was 1,125/.,
that of the masonry being about 1501. ; and it was
stated, to have continued in use, with perfect satis-
faction, since its completion in February, 1847.
This form ef construction was considered to he
applicable, in situations where a comparatively nar-
row water channel had to be crossed by a very wide
roadway; but as the particular circumstances of
other localities might differ from the one in question,
the author suggested various alterations in the, de-
tailes, so as to meet these exigencies.
The next paper read was a " Description of a
wrought-iron Lattice Bridge, constructed over the
line of the Rugby and Leamington Railway," by W.
T. Doyne, Assoc. Inst. C. E.
This bridge, which was 150 feet span, carried a
public road over the Honinghatn cutting. It con-
sisted of two girders 156 feet in length, and 10 feet
6 inches in depth, placed at a distance of 20 feet apart,
and connected together by means of wrought-iron

transverse girders, and by a :system" of horizontal
etii;gornil bracing. The bottotn c,f the ;min girders
were formed of two angle irons, and wrought-iron
plates, eight in number at the centre, but diminishing
to three at the ends, and of such dimensions as to
trmke the effective sectional area at the centre, after
deducting the loss by rivet holes, equal to 26 square
inches; that of the top, which was somewhat differ-
ently constructed, so as the better to resist compres-
sion, being equal to 40 square inches. The lattices
were formed of a series of bars of spoke-iron, intersect-
ing each other at an angle of 60 deg., being crossed at
those points, by longitudinal bars, for the purpose of
giving additional rigidity, and of making a closer
parapet. The transverse girders, 7 feet 6 inches
apart, were each formed of a plate of wrought iron,
with two angle irons at the top and the bottom ; these
were covered with corrugated galvanized iron, one-
tenth cf an inch thick, upon which concrete and then
a layer ( f gra% el and loam metalling, 6 inches thick,
were laid. This bridge was erected by Messrs.
Smith, Smi h, and James, of Leamington, Upon a
plalornt wi ich gave to the girders a camber of 7
inches in the centre, which was rcduced to.n inches
upon removing the tlatform. rine total cost Lf the
bridge was about 3 500/._ . .

Dui ing the progress of the works the author made
some experiments upon the strength of rivets of dif-
ferent siz-s, from which it appeared th:tt the average
breaking weight, per squart, itich of m ctional area,
was 35.10 tom for a chain joint, and 1S 82 tons fur a
lap joint.
The paper announced to be read at the next meet-
ing, Tuesday, April 301 h, when the monlly ballot
for members will take place, was No. 834, "On the
Absorbent Power of Chalk, and its Water Contents,
under different conditions," by Professor /kilted.