(Letter written Jul 2 1866, original publication date unknown)
A copy of another letter written by Matthew to the Daily Review on the Forth and Tay Bridges. He warns that subsidence of the Forth Bridge will be a problem, and that the railway companies put forward to run the Tay Bridge will conspire to put ticket prices up to maximise their profit. He ends by again pointing out the advantages of his alternative Tay Bridge scheme near Newburgh, which he had first advocated more than 30 years previously.
THE FORTH AND TAY BRIDGES.
(From the Daily Review.)
Sir,— Referring to the plan of the proposed bridge over the Firth of Forth, as given in your paper a few days ago, it appears to me that the engineers deceive themselves in some of their conclusions regarding this untried scheme, more especially in the expected stability of the piers. It was stated in your paper that the weight with which the wooden basement is primarily to be loaded, would sink it about seven feet into the silt. Subsidence in silt is an affair of time, and unless the weight was continued for a considerable length of time, the wooden platform would not sink nearly so far. Again, I expect in the case of the actual success of this bridge scheme, and trains running along it, that the shake or vibration thus produced will cause a continued gradual sinking, owing to the vibration deranging the capillary attraction between the earth and sand particles and the water composing the moist silt. In consequence of the shake the silt would become more fluid, as shaken lime-mortar does, and the earth part would subside and the water drain out; or that the silt below the timber, softened by the vibration, would subside under the incumbent pressure, and rise around the platform, as we observe in some old coal pits, where with much harder materials, the piers sink and the mined interstices rise till the mine becomes closed up. Say, as has been stated, that the depth of this is in some places, in the line of the bridge, 120 feet, we must expect that the bridge in the course of time will go far to burrow it, in mole fashion. When a little boy, 1 was wont to amuse myself dancing upon or shaking with my feet the sand by the side of the river six inches or more above the water level, and causing streams of water to run down from the shaken sand into the river. Here the shaking deranged the capillary attraction or fixing power, the sand sank down and the water rose from the difference of gravity, or rather the shaking, causing the small cavities or vacuums, acted as a pump to raise the water.
Although the Forth Bridge should be erected with passages upwards of 120 feet clear at high-water, they will not remain long of this height. A subsidence must ensue, and the bridge will require, from time to time, a re-elevation. It is also not to be expected that the subsidence will be always perpendicular. The curve line of the bridges will necessarily cause the subsidence to be greatest on the outer side of the curve, and should the rock bottom below the silt be sloping, one side of the basement will sink lower than the other, so as to upset the bridge, even in the case of one pier deflecting from the perpendicular. How is this to be guarded against?
I may also mention that it is not wise in the people of Dundee to trust much to an attempt to prevent monopoly of the means of conveyance. Such is the purely selfish character of companies, that the two railways, which they expect are to give competition, will soon see it their interest to amalgamate, or, at least, to enter into an agreement with regard to carriage and passenger charges, and raise the fares to the highest paying point, which experience will enable them to ascertain,— that is the highest that will not prevent more intercourse than overbalance the too high charge. Where there is not a maximum legislated, what will pay best is the natural charge, and will be forthcoming.
In pointing out these facts and some others to the people of Dundee and the shareholders of the North British, I run the risk of being regarded as an antagonist to progress, when it is only being perhaps a little too earnest and sanguine in improvement that impels me to point out the folly of a scheme next to if not entirely impracticable, and to point out in the case of the Tay, one that is practicable, at one-third of the probable cost of the other, and equally if not more advantageous to Dundee. A bridge at Dundee will never, either in cheapness or convenience, equal a water steam-conveyance across. Near an age ago, a number of years before the Perth and Dundee Railway was formed, I pointed out the importance and practicability of a bridge near Newburgh, at the west end of the Tay Firth, as the natural position for a bridge, stating the great advantage this near access to the coal-fields of Fife would afford to Dundee, Arbroath, &c., and that this bridge would serve at the same time the north and south and east and west traffic of Scotland. This has been admitted be correct, and only not carried out from the rivalry of railway companies, and the erroneous views of directors. The actual result will prove whether I have been right or wrong.— I am, &c.,
Gourdiehill, July 2, 1866.