Part 20 (1/2)
The performances of nearly all the pumping-engines in Cornwall were for many years so systematically and exactly reported, and the reports of each were so critically scrutinised by the rival makers, that the data they supply may be relied on without hesitation. It was well known that the best of the engines continuously performed useful work with a consumption of coal at the rate of 233 lbs. per delivered horse-power per hour, or, counting coal at 16_s._ per ton (a fair price on the South Devon), at the cost of 2_d._, or one-fifth of a penny per horse-power per hour.
But it was not in its consumption of fuel alone that stationary power was the more economical; the expenditure in wages, oil, and tallow on one of the pumping-engines above referred to, when doing 200 horse-power of useful work, did not exceed 20_s._ for the twenty-four hours, or one-twentieth of a penny per horse-power per hour, while the cost of repairs was merely nominal.
Thus if fuel, wages, oil, and tallow be brought into one item, it is seen that the cost of one horse-power in stationary engines such as the then existing Cornish engines was only 25_d._ per hour, or less than one-fourth of its cost when developed by a locomotive, which has been shown to have been 1098_d._ per hour.
[Ill.u.s.tration: PLATE III
THE ROYAL ALBERT BRIDGE.
H. Adlard. Sc.]
CHAPTER VII.
_RAILWAY BRIDGES AND VIADUCTS._
1. BRICKWORK AND MASONRY BRIDGES--HANWELL VIADUCT--MAIDENHEAD BRIDGE--FLYING BRIDGES--LETTER FROM MR. BRUNEL ON BRIDGE CONSTRUCTION (DECEMBER 30, 1854)--2. TIMBER BRIDGES--SONNING BRIDGE--BATH BRIDGE--STONEHOUSE VIADUCT--BOURNE VIADUCT--ST. MARY'S VIADUCT--VIADUCTS ON THE SOUTH DEVON RAILWAY--IVY-BRIDGE--VIADUCTS ON THE SOUTH WALES RAILWAY--NEWPORT--LANDORE--VIADUCTS ON THE CORNWALL RAILWAY--ST. PINNOCK--VIADUCTS ON THE WEST CORNWALL AND TAVISTOCK RAILWAYS--PRESERVATION OF TIMBER--3. CAST-IRON BRIDGES--LETTER ON USE OF CAST IRON (APRIL 18, 1849)--HANWELL BRIDGE--EXPERIMENTS ON CAST-IRON GIRDERS--EXTRACT FROM LETTER TO SECRETARY OF COMMISSION ON APPLICATION OF IRON TO RAILWAY STRUCTURES (MARCH 13, 1848)--4. WROUGHT-IRON BRIDGES--GIRDER BRIDGES--EXPERIMENTS ON WROUGHT-IRON GIRDER--OPENING BRIDGES--TRUSSED BRIDGES--NEWPORT VIADUCT--WINDSOR BRIDGE--CHEPSTOW BRIDGE--METHOD OF SINKING THE CYLINDERS--DESCRIPTION OF THE MAIN TRUSS--THE FLOATING OPERATIONS--THE ROYAL ALBERT BRIDGE AT SALTASH--THE CENTRE PIER--DESCRIPTION OF THE SUPERSTRUCTURE--THE FLOATING AND RAISING OF THE TRUSSES--OPENING OF THE BRIDGE BY H. R.
H. THE PRINCE CONSORT--_NOTE_: EXPERIMENTS ON MATTERS CONNECTED WITH BRIDGE CONSTRUCTION.
In Chapter IV. a general history has been given of the railways of which Mr. Brunel was the engineer; but the bridges and viaducts designed by him are so numerous and important that it has been thought advisable to devote a separate chapter to their consideration.
The bridges selected for mention have been grouped according to the nature of the material used in their superstructure. This arrangement is the most convenient one for giving a concise description of the most remarkable of Mr. Brunel's bridges, and for stating the circ.u.mstances which guided him in the determination of the particular form of construction used in each case.
The works are therefore divided into four groups, namely, brickwork and masonry, timber, cast iron, and wrought iron.[77]
_Brickwork and Masonry Bridges._[78]
The viaduct which carries the Great Western Railway over the valley of the river Brent near Hanwell is the first of Mr. Brunel's important railway works.[79] It is a handsome brickwork structure, 65 feet high, with eight semi-elliptical arches, each 70 feet span and 17 feet 6 inches rise. The spandrils of the arches are lightened by longitudinal spandril-walls; the piers are also hollow, and the structure is throughout made as light as possible. It is on this account interesting, as showing the care taken by Mr. Brunel from the commencement of his practice to distribute the material in the simplest and most effective manner.[80]
The great bridge over the Thames at Maidenhead contains two of the flattest, and probably the largest arches that have yet been constructed in brickwork. The river, which is about 290 feet wide, flows between low banks; in the middle of the stream there is a small shoal, of which Mr.
Brunel took advantage in building the centre pier.
It was originally intended that the foundation of the bridge should be on the chalk, which was at a short distance below the surface; but it was found to be very soft, and Mr. Brunel therefore decided to place the foundations of the bridge on a hard gravel conglomerate overlying the chalk. The main arches are semi-elliptical, each of 128 feet span and 24 feet 3 inches rise. They are flanked at each end by four semicircular arches, one of 21 feet span, and three of 28 feet span, intended to give additional water-way during floods. The radius of curvature at the crown of the large arches is 165 feet, and the horizontal thrust on the brickwork at that point is about 10 tons per square foot.
In the interior of the structure immediately landward of the large arches, Mr. Brunel constructed flat arches loaded with concrete. The centerings of these were struck, and an active thrust opposed to the main arches before their centerings were eased.[81] The line of pressure of each main arch was diverted downwards by the thrust of the flat arch adjoining it without the necessity of employing a great ma.s.s of brickwork in the abutment.
The woodcut (fig. 1) shows the form of the main arches and the flat arch referred to.[82]
[Ill.u.s.tration: Fig. 1. Maidenhead Bridge.
_Longitudinal Section._
_Scale of feet._]
The Maidenhead bridge is remarkable not only for the boldness and ingenuity of its design, but also for the gracefulness of its appearance. If Mr. Brunel had erected this bridge at a later period, he would probably have employed timber or iron; but it cannot be a matter of regret that this part of the Thames, although subjected to the dreaded invasion of a railway, has been crossed by a structure which enhances the beauty of the scenery.