The Sun.
London, England, 1847-07-09 00:00:00

COAST OF IRELAND.

The Comet, steam-sloop, Lieutenant• Commander
Johnstone.
The Stromboli, steam-sloop, Commander Fisher.

WORKS OF NAVIGATION IN
IRELAND CONNECTED WITH DRAINGE.
(Presented by Command of her Majesty.)
AN ESTIMATE of the further Sum proposed to be
voted in the Year 1847, to defray One Moiety of the
Cost of executinc, certain Works of Navigation in
Ireland connected with Drainage.
Five Thousand Five Hundred Pounds.
Sum proposed to be applied by way of Grant for
the Improvement of the Navigation of the
River Hind, in the County of Roscommon,
Ireland, under the provisions of an Act
of the last Session of Parliament (9th of
Victoria, cap. 4, section 33), intituled " An
Act to amend the Acts for promoting the
Drainage of Lands and Improvement of Na-
vigation and Water Power in connexion with
such Drainage in Ireland, and to afford
facilities for increased Employment for the '
Labouring Classes in Works of Drainage"... £5,500
An Estimate of £33,000 for Works of Navigation in
Ireland connected with Drainage appears in the Seventh
Class, No. 3. J. PARKER.
Whitehall, Treasui:y Chambers,
June 22 1847.

THE TUNNEL OVER THE MENAI

STRAITS.
TO THE EDITOR OF THE SUN.

RESPECTED FRIEND,—I regret I was not in time to
get an opportunity of bringing my several subjects before
the. Association in that form which appeared to me ad-
visable ; I, therefore, preferred to leave it to a future
opportunity. I was much interested in hearing, at the
Mechanical Section, the description of Clarke and Var-
ley's atmospheric tube, which I think is the most effec-
tive plan of the kind I have yet seen. I was 'permitted
to exhibit my machine for proving the strain on beams,
bridges, &c., in the society's model room, where it was
seen by many of the professors, &c., and its simplicity
and accuracy was admitted •by all. By a professor of
this section I was informed some of the particulars of the
proposed iron tunnel, or tube, for conveying a railway
over the Menai Straits ; and though I was very much
surprised at the boldness of constructing cast-iron beams,
or girders, 100 ft. long, and less than 4 ft. deep, I was
still more astonished to hear of the amazing boldness of
constructing a tunnel, or tube, of such magnitude, with
such comparatively slight materials. According to my
informant, its length is to be 460 ft.., clear of the points
of bearing—so that I presume the entire length cannot
be far short of 470 ft., its diameter 30 ft., to be con-
structed with wrought-iron plates, varying from in.
to iI in. thick—therefore, supposing the average thick-
ness to be a. and including the needful powerful ribs and
stays to resist the vertical and literal pressures and
strains, together with the interior framing and rails, I
estimate its weight at upwards of 30 cwt. for
every foot of its length, or about 700 tons. Now the
depth of the tube is 30 ft., which is about 1-15th part
of its
length ; therefore, according to my machine, the
strain by its own weight at the middle will be equal to
7i times its weight—viz., 5,250 tons ; and suppose a
train, with the engine and load, to be 100 tons, equal to
750 tons—together 6,000 tons. According to Barlow's
theory, the safe capacity to resist such a strain ought to
be equal to 18,000 tons, and to Tredgold's, 24,000 tons.
Now let us see what is the capacity of this projected
tunnel, or tube:—
The circumference is (say) 90 ft. ; the tension pro-
perty in the lower half, 45 ft. Suppose the plates average
5-Bths of an inch thick, it would give 7i square inches
on each foot ; say that each inch would sustain, before
breaking, 20 tons, this would make 150 tons to each foot,
which, multiplied by 45, gives a capacity of 6,750 tons I
—so that its own weight, and the load, would, according
to this calculation, produce a strain nearly equal to the
breaking point. Now, as wrought-iron will begin to
stretch at the strain of 10 or 12 tons to the inch, it must
be manifest that little more than half its weight would
produce a permanent set, and render it perfectly unfit
and unsafe for the desired object.
If this projected tunnel, or tube, is actually in progress
of construction, or is seriously intended to be made, I
cannot but think it would be advisable for the parties
concerned to have the matter or subject investigated, so
as to prevent the probability of incurring great pecuniary
loss, as well as to guard against the pos.ibility of a great
sacrifice of human life. The foregoing calculation is
upon the assumption that the entire support is intended
to be derived from the tension of the wrought-iron plates;
and I do not see on what else they can depend, unless it
may be intended to suspend the tube from towers of
masonry. In that case, I cannot but consider it a most
expensive mode for obtaining the object—far greater than
the system of triangular bracing, or straight suspending
bars. T. MOTLEY, C.E.
Oxford, 7 mo., 1.