674
THEORY AND PRACTICE OF ENGINEERING.
Book II.
by the miner is commonly not one of continuous descent and ascent, but varied by histraversing at different intervals a considerable length of horizontal galleries.
Mr. De la Beche in his geological report has estimated the value of the mineral exportedproduce of Cornwall and Devon in 1837, as follows: —
Copper ------- 952,855
Manganese - - - - - 40,000
Lead ------- 3,000
China-stone and clay - 43,000
Granite ------- 21,500
£ 1,478,873
The smallest height of the levels in Carnon mine from which a horizontal bed of tinstuff is removed is 5 feet, and the thickness of the vein of ore 3 feet; and the ore is about13 fathoms from the surface of the ground.
St. Ive’s Consols well illustrate the character of the greater part of the tin mines. Thesmallest height of the levels is seldom less than 6 feet; the thickness of the bed or vein ofore is from a few inches to 10 feet, and sometimes more than double that in width. Thelodes in this district enter the rock at the surface, at an angle of 50°, 60°, or 70°, from thehorizon, and sometimes almost vertically, and if productive, from any given level toanother, are regularly cut away, after first supporting the sides and roof of the levelwith timber frame-work fitted to the angle of the lode, which in few cases proves otherwisethan completely safe and secure. The workings below the level or surface of the groundvary from 30 to 147 fathoms, and 20 fathoms from the adit.
In the Charlestown tin mines the smallest height of the levels is 7 feet, and the thick-ness of the bed or vein of ore is from 3 to 10 feet. In the copper mines in the centraldistrict, in the United Mines for instance, the levels in the ancient workings do notexceed 5 feet high and 2 feet wide; but those made recently are about 7 feet high and4 feet wide. The veins are nearly perpendicular, and vary from 1 inch to 9 feet wide:the ores are got from between 40 to 220 fathoms from the surface. In the ConsolidatedMines of this district, the deepest, the smallest height of the level is 6 feet, and width2£ feet: the openings in the platform, from one ladder to the other, 18 by 12 inches; thethickness of the vein of ore varies considerably, sometimes being 8 feet, at others a fewinches only. The veins do not incline much from the perpendicular, and consequently thelevels are driven 6 feet high, and the ground above is worked afterwards. The ore is sometimesfound nearer the surface than the adit level; but in general it is worked from 20 fathomsfrom the surface, to the 260 fathom level below the adit, the deepest point being nearly300 fathoms from the surface.
In the large copper mines of the Levant in the western district, the height of all thelevels is 6 feet, and from thence to the next level 10 fathoms or 60 feet; the thicknessor width of the whole vein, where the ore is found, is on an average about 4 feet. Thevein, almost perpendicular, having a small declination only, is worked by the side at first,and taken down afterwards. The adit or sea level is 30 fathoms under the surface, andthe ore in work from 70 to 230 fathoms below the adit.
In the Eastern Cornwall district, the Fowey Consols copper mine is the most con-siderable ; the smallest height of the levels is not less than 6 feet, and often 7 feet or more,where air-pipes are required for ventilation. There are no horseways in these Cornishmines; the twenty lodes vary in thickness from 8 feet to only a few inches. 'When thelodes are perpendicular, and of a sufficient size for the levels to be driven, they do not cutaway any of the overlay or underlay, as the lode is very rarely perpendicular. Theair of these shafts and levels is more condensed than that on the surface, with atemperature higher in proportion to the depth. There is no reason to believe that anygas except carbonic acid is generated from the strata in these mines: where they havebeen carried beneath beds of alluvium, which are periodically submerged, some of theinflammable compounds of hydrogen are at times emitted. The natural temperature atdifferent depths, and in different strata, is given by Mr. Hcnwood, who personallyinspected 200 mines in Cornwall and Devon, and made several hundred observationson the temperature of the streams of water which flowed from the unbroken rocks.
Depth in Fathoms.
In Slate.
Surface to 50
.
- 57° Fahr.
-
-
- 51*6°
50 to 100
-
.
- 61-3
-
-
- 55-8
100 to 150
-
-
- 68
-
-
- 65 ’5
150 to 200
-
.
- 78
200 and upwards
-
-
- 85*6
-
-
- 81-3
When work is carried on, there is of course a rapid exchange of oxygen for carbonicacid, by means of the respiration of the miners, the burning of candles, and the blasting