The times., 1901-03-17

| ing was perfectly controlled, and the cais-
son finally placed In its exact position,
| and perfectly vertical. The internal space
! was then filled with concrete laid under
water by the same bucket, and leyeled by
| divers when necessary.
| While this work was going on, the Gov
ernment of New South Wales in Australia
| called for both, designs and tenders for
| bridge over an estuary of the sea called
Hawkesbury The conditions were the
same as at Poughkeepsie, except that the
soft mud reached to a depth of 100 feet
| below tide-level
The designs of the engineers of the
| Poughkeepsie bridge were accepted, and
| the same method of sinking open cais-
sons lin this case made of iron) was car
| ried out with perfect success.
| The erection of this bridge involved an
| other difficult problem. The mud was too
| soft and deep for piles and staging, and
| the cantilever system in this site would
| have increased the cost.
| A staging was built on a large pontoon
| at the shore, and the span erected upon
! it. The whole was then towed out to the
| bridge site at high tide. AS the tide fell,
the pontoon was lowered and the steel
girdcr was placed gently on it's piers. The
| whole operation was completed within SIX
| hours. The other five spans were placed
| in the same manner.
The same system was followed after
| ward by the engineer of the Canadian
Pacific Railway in placing the spans OF
| bridge over the st. Lawrence, in very
| rapid current 1t is now used in replac-
ing old spans by new ones, as it interrupts
| traffic for the least possible time.
| The solution of the problems presented
| at Hawkesbury gave the second intro
| duction of American engineers to bridge
| building outside of America. The first
was in 17S6, when an American carpenter
or shipwright built a bridge over Charles
River, at Boston, 1,47O feet long by forty
| six feet wide. This bridge was of wood
| supported on piles. His work gained for
him such renown that he was called to
Ireland and built a similar bridge at Bel
! fast.
Tunneling by compressed air is hori-
zontal application of compressed air foun-
| dations. The earth is supported by an
iron tube which is added to in rings,
| which are pushed forward By hydraulic
jacks.
| a tunnel IS no" DelnS tnauS uHuef an
arm of the sea between Boston and East
Boston, some l. 400 feet long and sixty
| five feet below tide. The interior lining
of iron tubing is not used. The tunnel
| is built OF concrete. REINFORCED by steel
| rods. This will effect a considerable
| economy. Success in modern engineering
means doing a thing in the most econom-
ical way consistent with safety.
The st. Clair tunnel, which carries the
| Grand Trunk Railway of Canada. under
the outlet of Lake Huron, is a successful
| example of such work. Had the North
| River tunnel, at New York, been designed
on equally scientific principles, it would
probably have been finished, which now
seems problematical.
The construction of rapid-transit rail-
| ways in cities is another branch of en-
gineering, covering structural, mecheni-
cal and electrical engineering. Some of
these railways are. elevated, and are mere
ly railway viaducts but the favorite type
now is tfRt Of subways. There are two
kinds, those near the surface, like the dis
| trict railways of London, the subways in
| Paris. Berlin. and Boston, and that now
| building in New York. The South London
| and Central London, and other London
| projects, are tubes sunk fifty TO eighty
feet below the surface and requiring ele-
| vators for access. These are made on n
| plan devised by Greathead, and consist
| of cast-iron tubes pushed forward by hy-
| draulic rams, and having the space out
side Of the tube filed with liquid cement
pumped into place.
The construction of the Boston subway
! diai,,,It ~, account Of tho small xyi5t n
of the streets. their great traffic, and the
necessity of underpinning the foundations
of buildings. All of this was successfully
done without disturbing the traffic for
A single day, and reflects great credit on
the engineer. Owing to the great width
of New York streets, the problem is sim
pier in that respect, but requires skill in
design and organization TO complete the
work in a short time Although many
times as long as the Boston subway, it
will be built in nearly the same time. The
design, where in earth. may be compared
TO that of a steel Office building twenty
miles long laid fat on one of it's sides.
The reduplication Of parts saves time and
labor. and is the key to the anticipated
rapid progress. Near the surface this
subway is built in open excavation, and
| tunneling is confined to rock.
The construction of power-houses for
developing energy from coal and from
falling water requires much structural be-
sides electrical and mechanical engineer
ing ability. The Niagara power house is
intended to develop 100000 horse power;
that at the Sault Ste. Marie as much; that
on the St. Lawrence, at Massena, 20 900
horse power. These are huge works, re-
quiring tunnels, rock-cut chambers, and
masonry and concrete in walls and dams.
They cover large extents of territory
The contrast in size of the coal-using
power-houses is interesting The new
power-house now building by the Manhat-
tan Elevated Railway. in New York, de-
yeiops in the small space Of 200 by 400 feet
100 900 horse power, or as much power as
that utilized at Niagara Falls.
one Of the most useful materials which
modern engineers now make use of is
concrete, which can be put into confined
spaces and laid under water 1t costs less
than masonry, while as strong. This is
the revival of the use of material used
by the Romans. The writer was once al
lowed to climb a ladder and look at the
construction Of a dome Of the Pantheon,
at Rome He found it a monolthic mass
Of concrete and hence without thrust.
II is a better piece of engineering CON
struction than the dome of st. Peters,
built fifteen hundred years later. The
dome of Columbia College Library in New
York is built Of concrete.
Concrete is mixture of broken stone
nd aud pnrtIa nd ferDaDt =