Mallet’s Insistent Pontoon

In my page on the Broadstone, I said:

In 1845, the Midland Great Western Railway Company (MGWR) bought the Royal Canal. Doing so allowed it to run its lines beside the canal, which it did most of the way to Mullingar, without having to conduct lengthy negotiations about wayleaves with individual landowners. The company built a terminus at the Broadstone, with a pontoon bridge (which was moved out of the way when boats entered or left the harbour) to provide passengers with access to the station.

In 1877, the company did away with that nuisance by filling in the harbour and adapting the Foster Aqueduct to provide access to the station from a new road, later called Western Way. The canal traders were provided with a turning circle and a wharf on the east side of the former aqueduct and they no longer used the Broadstone.

The two historic Ordnance Survey maps available online show the situation before the MGWR bought the Royal Canal …

The Broadstone ~1840

The Broadstone ~1840

… and after the harbour was filled in in 1877 …

Broadstone ~1900

The Broadstone ~1900

… but they don’t show the period in between, when the railway station and the harbour shared the same space and when the pontoon bridge was in operation. I’ve done a mock-up by pasting an extract from the later map on top of the earlier:

Broadstone ~1840 edited

The Broadstone 1845–1877 with both railway station and harbour

You can see that the MGWR’s impressive station building …

The Broadstone building

The Broadstone building

… is extremely close to the canal, much closer than you might think from the 1854 illustration.

The Broadstone, from The Tourist's Illustrated Hand-Book for Ireland 3rd ed David Bryce, London 1854

The Broadstone, from The Tourist’s Illustrated Hand-Book for Ireland 3rd ed David Bryce, London 1854

You might ask why they didn’t build it a little further away; I don’t know the answer to that. But I have recently come across an account of the design of the pontoon bridge; it makes clear the nature of the problem that the pontoon was intended to solve. The account is in a paper read by Robert Mallet to the Institution of Civil Engineers on 23 April 1850 [Min Proc Inst CE 9 (1850) 344-352], with William Cubitt in the chair.

The problem

First, although the MGWR was permitted to buy the Royal Canal, it was forced to keep the navigation open. And because its lines were to run along the canal banks for most of the way to Mullingar, the Broadstone terminus had to be at the same level as the harbour, which was on an embankment 30′ [9.14 m] above the Phibsborough Road.

That meant keeping the station buildings north-west of the harbour, but in turn that meant that passengers and vehicles, arriving and departing, would have to cross the canal to get to the station. Because lots of people would need to cross at the same time, before or after a train arrived, it was felt that a 50′ [15.24 m] wide bridge would be needed. The canal at the chosen crossing point was 17′ 4″ [5.28 m] wide and 8′ [2.44 m] deep.

Not a swivel bridge

Mallet was asked to design a swivel bridge, but he found two problems. First, the water was only 16″ [0.41 m] below the bank, which would have left little room for construction, and, second, the 50′ [15.24 m] deck would obviously have needed 50′ of space to swivel back into, but there was only about 20′ [6.10 m] available.

At the MGWR’s request, he then designed two parallel bridges, each 25′ [7.62 m] wide, and a short distance apart, and got tenders for the construction from the family firm, John and Robert Mallet’s Victoria Foundry, which was initially at Ryder’s Row and later (I don’t know when) moved to the banks of the Royal Canal at Cross Guns Bridge, just upstream of the junction of the Broadstone Line with the main line of the Royal Canal.

Ryder's Row

Ryder’s Row

A better idea

In the meantime, though, Mallet had a better idea: to put a deck on a boat and position it between the banks of the canal. He may have been inspired by the fact that the space to be bridged, 50′ X 17′ 4″, resembled the proportions of a canal boat.

He designed a fairly shallow flat-bottomed iron boat, 50′ 6″ [15.39 m] X 16′ 8″ [5.08 m] X 2′ 10″ [0.86 m]. It had a timber rubbing-strake along each side.

It carried a deck made of two layers of planking, both caulked and tarred and the upper layer sanded. There was a wrought-iron handrail at each end, then a five-foot-wide [1.52 m] “footway” for pedestrians; a “cast-iron wheel-guard of the ordinary form” [ie a kerb] separated the footways from the carriage-way.

The deck was 16″ [o.41 m] wider than the pontoon so that it overlapped the two sides of the canal; it was designed to have a clearance of 2″ [0.05 m] above the sides so that the pontoon could be moved easily. A shallow “lye by”, just 3′ [0.91 m] deep, was built into which the pontoon was moved when boats needed to pass along the canal; Mallet said that the pontoon could be lifted out from the lye-bay, using blocks and screw-jacks, for maintenance or painting.

The lye-by

The lye-by

Insistent

Mallet referred to his device as an insistent pontoon bridge, but didn’t explain what he meant by the term. There are two meanings of insistent that might be relevant. One is unyielding, and Mallet certainly felt that it was undesirable that the pontoon should move under the weight of traffic:

[…] a floating pontoon would afford a very unstable and inconvenient surface for wheel carriages, &C., to traverse […].

The other sense is from ornithology:

(of a bird’s hind-toe) touching the ground with the tip only. [The Chambers Dictionary 10th ed, Edinburgh 2006]

As we have seen, Mallet’s pontoon bridge was designed to float with the deck 2″ [0.05 m] above the sides of the canal, but he wanted it to be fixed in position when in use.

Accordingly, Mallet designed a way of taking on water ballast, when the pontoon was to be fixed in position, and of getting rid of it again, when the pontoon had to be moved to its lye-by. He also provided shallow rabbates [nowadays rabbets or rebates] in the two sides of the canal to hold the pontoon in position and to make the deck level with the banks.

Taking on water ballast was easy: he put a large valve at each end in the bottom of the pontoon. Once it was in position above the rabbates, the valves could be opened and water would flow in until the deck rested on the rabbates. The valves could then be closed. The valves, operated from deck level, were under cast-iron covers in the footways; you can see the small circles in the middle of the footways on the drawing above.

Getting the water out again required rather more ingenuity: Mallet used the principle of the syphon [also siphon]. If you look again at the drawing above, you’ll see that the right-hand footway has a second circle at the bottom and that there is a device of some kind on the bank beside it. This second circle is the intake of the syphon; the inside floor of the pontoon sloped slightly towards that point so that water would flow towards the syphon.

You’ll also see a bell-ended pipe extending from the canal to the device on the bank. Opening a valve or cock took water from the canal and got the flow going; that flow in turn started the syphon effect, which extracted water from inside the pontoon. Once that happened, the flow from the canal could be cut off.

Mallet used the fact that the canal was 30′ [9.14 m] above the Phibsborough Road to provide the difference in height required for the operation of a syphon. The only remaining problem, then, was that a flexible pipe would be needed to make a temporary connection between the part of the mechanism on the pontoon and the part on the bank. Mallet used a hose:

[…] an intermediate flexible tube, formed of a spiral of hammer-hardened hoop iron, 6 inches in diameter.

The hoop iron was covered by a sheet of gutta-percha, covered in turn by a leather coat to provide protection. Wikipedia says that …

Western inventors discovered the properties of gutta-percha latex in 1842 […].

… which lends credence to Mallet’s claim that …

[…] the Author believes, that this was the first occasion upon which a continuous lining of gutta percha was thus applied, to any suction pipe, and he has found it to answer so perfectly that he has since applied it largely, and with success, to the suction-pipes of fire-engines.

How well did it work?

The whole arrangement sounds very elaborate, but Mallet said that it worked well. It cost £1125, plus about £150 for masonry work, and was built between October 1846 and February 1847. It had been in continuous use up to December 1849 [when Mallet wrote his paper], “giving perfect satisfaction”.

It needed only one man to operate it. Pumping out the water took only about three minutes; the whole operation of shifting it out of the way, to leave the canal open, took four minutes, and in three minutes the bridge could be back in position. As Mallet put it:

These operations may probably appear slow and troublesome, but the fact is, that the bridge in question, which is twice the width of roadway of any swivel bridge in Great Britain, is readily opened, leaving the space free for navigation in four minutes, and it can be closed again, and the roadway passage be made complete, in less than three minutes; being much less time than is usually required for moving common swivel bridges; and, if it was desirable, the time could be easily reduced.

As fas as I know, the pontoon arrangement lasted the thirty years from 1847, when it was built, until 1877, when the harbour was filled in, but it would be interesting to know more about its use in the intervening years.

If you are down wid da kidz in da technological hood, and know what QR Codes and NFC Tags are, and have one of those newfangled telephone devices, be aware that you get get more information as you visit some places along the Royal Canal; and if you’re still, like me, trying to get to grips with the non-existence of telephones with dials, you can print information in advance from Canals of Dublin, which explains all about the scheme.

My OSI logo and permit number for website

[broadstone 10]

5 responses to “Mallet’s Insistent Pontoon

  1. Fascinating, thank you.

  2. Your old stamping ground, of course! bjg

  3. Pingback: Vanished Dublin: Royal Canal Harbour | Sibling of Daedalus

  4. Pingback: Robert Mallet | Canals of Dublin

  5. Pingback: The lost buildings and monuments of Ireland - Page 27

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