694
THEORY AND PRACTICE OF ENGINEERING.
Book II.
The superficial content of the middle s^an is 10,235 feet, and that of the two side roofs10,810 feet, each being a little more than half that in the centre. As the weight of thiscorrugated iron is 3pounds per superficial foot, the whole weight is 28J tons: the cost oferection was 61. 10s per square of 100 feet, or 1365/.
The water is carried off from the roof down the curves of corrugation, first into the gutter,then through the hollow columns, and afterwards by drains. A single sheet of this iron wasfound to bear 700 pounds weight in a vertical position without bending. Many otherroofs of this description have been executed, and apparently stand well; one at the London docks is 225 feet in length, and 40 feet span. St. Catherine’s Docks, the Birmingham ,Blackwall, and numerous other railways, have made use of them.
Galvanised iron y as it has been termed in France , is made by covering the metal with acoat of tin by a peculiar process, and for a time it resists the corrosive effects of the atmo-sphere as well as that of water : the surface of the iron is first rendered perfectly clean by thejoint action of dilute acid and friction; after which, it is plunged into a bath of melted zinc,and moved about until entirely covered with the alloy; it is then taken out and immersedin a bath of tin, which covers it with a thin coat of alloy. It is stated that when iron thusheated is exposed to humidity, the zinc slowly oxidises, and protects the former from rustingwithin it whilst the outer tinned surface remains.
Coal is found in many parts of the British Islands. The culmiferous series in Devonshirelies in a great basin, the axis of which extends 50 miles from east to west, with an averagebreadth of 30 miles; the upper beds of slate of the Devonian system are occupied by darkcoloured limestone, over which occurs a stratum of siliceous flagstones ; over these are sand-stones, carbonaceous and calcareous slates, which are surmounted by a bed of thicksandstone.
The South Wales coal field extends about 90 miles along the shores of the Bristol Channel ; its greatest breadth is not more than 20 miles; the number of bands of coal isconsiderable; their thickness varies from 18 inches to 9 feet, and the whole taken togetheramount to a depth of 95 feet: in the deepest part they He about 13,000 feet below thesurface.
The Somersetshire and Bristol coal field is of small extent; that of South Staffordshirehas only eleven seams, but the main bed in the middle is upwards of 30 feet in thickness;this seam crops out near Bilston. The coal in the northern portion contains numerousimpressions of plants.
The Shrewsbury coal field is not very extensive : that in Flintshire is under the new redsandstone ; this latter is about 40 miles in length, and 3 in breadth. But the most im-portant of all are those in the north of England, which form three districts : in the first iscomprised Yorkshire , Derbyshire, and Nottinghamshire ; in the second, Lancashire ; andin the third, Durham and Newcastle .
In the first the beds vary from 2 to 5 feet in thickness, and are of a bituminous qua-lity. In the Lancashire district, the extent from north to south is 46 miles, and about40 in width : in some parts there are 75 seams, forming altogether 150 feet of workablecoal.
Coal Fields. —The chief in England is the Newcastle , and lies between the rivers Coquetand Tees, its length being nearly 50 miles, and its breadth upwards of 20; the area of thisdistrict, so important to British trade and manufactures, is computed at 800 square miles.It is divided by a great fault, which crosses it north of the Tyne, w’here the strata arethrown downwards on the one side, and uplifted to a height of 90 fathoms on the other ;this fault is termed the main dyke. The most valuable working is the high main, wherethe coal is 6 feet in thickness.
Boring is first resorted to for the purpose of ascertaining the best position for sinking theshaft by which the coal is to be drawn up ; this is performed in the ordinary way by meansof successive iron rods and machinery to work them, the cost of which is 12 shillings perfathom for the first ten, and an additional 6 shillings for each 5 fathoms beyond.
The shafts are cylindrical, and seldom less than 10 feet in diameter ; these are divided bya wall; some of the larger shafts are formed into three compartments, one of which is usedfor ventilation, another for drainage, and the third for drawing up the coal. Great expenseand caution are necessary in the works appertaining to this part of the operation ; thewhole of the shafts require to be cased or lined with good bricks or stone, and where thesprings are abundant there must be a tubing or a crib formed of whole deals attached tocircular ribs or curbs; metal castings are now sometimes substituted, as better calculatedto resist the extraordinary pressure to which the curb is subjected.
These shafts commonly extend to the depth of 150 feet, and sometimes as much as 1800;that at the 'W’earmouth Colliery, near Sunderland, passes through the capping of magnesianlimestone, the lower beds of which, with the lower new red sandstone, overlap the coalmeasures. After the shaft had been sunk 330 feet, the workmen tapped a spring, whichpoured out 3,000 gallons of water per minute ; this, however, being subdued by theworking of a steam-engine of 200 horse-power, a strong metal cylinder was introduced, andcarefully placed around the shaft; the sinking was then continued to the depth of 1578