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STONE: NATURAL AND ARTIFICIAL.
97
In tamping the headings, the powder was often built in for a few feet with
a dry rubble wall, and the remainder of the galleries rammed with clay,
obtained from a decomposed porphyry dyke in the quarry. The shafts were
more readily tamped with quarry débris, stones and clay thrown in and
rammed. Occasionally, especially at the outset, a space was left round the
charge, but it is believed that this was of little use.
The cost of quarrying the rock, including driving, powder and sundries,
was 4Jd. per ton, and the cost of filling into waggons, including blasting the
large stones, was about the same. The prices, however, refer to the period of
Depth of shaft, 44 feet.
Line ofleast resistance, 21 feet.
Charge, 900 Ibs.
Produce, 2000 tons.
Fig. 86. —Mine at Holyhead.
Height of Face . . 110 feet.
Length of Face . . 140 ,,
Length of Heading . 89 ,,
Grip of Heading . . 35 ,,
Depth of Sink . . 134
Chambers.
No.l. No. 2. No 3. No.4.
Length 121' 23*' 12' 9' 4' 18'
Line of least résistance 24' 25'
Charge in Ibs. 4,500 4,000 3,000 1,500
Produce, upwards of 60,000 tons.
Fia. 87.—“ Rooter-out”
Mine at Holyhead.
maximum output. The cost of driving the headings ranged from 10s. to 25s.
per lineal foot, out of which the miners had to pay, on an average, about 2s.
for powder, fuses, etc. The average length of heading driven was 5 feet per
week with four men employed.
Concrète.
The subject of natural stone leads on almost insensibly to the kindred
theme of artificial stone, for which an equally valuable, and a practically
unlimited, field of usefulness exists. Those parts of maritime structures
which are by far the most important and most prominent, are now constructed
in concrete, in place of the elaborate masonry which characterised and
distinguished the operations of past generations of engineers.
The cause and reason for this is not far to seek. Blocks of stone of large
size are difficult to procure, expensive to dress, and equally expensive to convey
and set in position. Smaller stones involve a multiplicity of joints. These,
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