the usual way and concreted
at the back, a small quantity of caustic soda being sometimes used in
mixing the concrete to prevent freezing. In an application of this
method at Vicq, two shafts of 12 and 16.4 ft. diameter, in a covering of
cretaceous strata, were frozen to a depth of 300 ft. in fifty days, the
actual sinking and lining operations requiring ninety days more. The
freezing machines were kept at work for 200 days, and 2191 tons of coal
were consumed in supplying steam for the compressors and circulating
pumps.

The introduction of these special methods has considerably simplified
the problem of sinking through water-bearing strata. Some of the earlier
sinkings of this kind, when pumps had to be depended on for keeping down
the water, were conducted at great cost, as, for instance, at South
Hetton, and more recently Ryhope, near Sunderland, through the magnesian
limestone of Durham.

Size of shafts.

The size and form of colliery shafts vary in different districts. In the
United States and Scotland rectangular pits secured by timber framings
are still common, but the tendency is now generally to make them round,
20 ft. being about the largest diameter employed. In the Midland
counties, from 7 to 9 ft. is a very common size, but larger dimensions
are adopted where a large production is required. Since the accident at
Hartley colliery in 1862, caused by the breaking of the pumping-engine
beam, which fell into the shaft and blocked it up, whereby the whole of
the men then at work in the mine were starved to death, it has been made
compulsory upon mine-owners in the United Kingdom to have two pits for
each working, in place of the single one divided by walls or brattices
which was formerly thought sufficient. The use of two independent
connexions--whether separate pits or sections of the same pit, between
the surface and the workings--is necessary for the service of the
ventilation, fresh air from the surface being carried down one, known as
the "downcast," while the foul or return air of the mine rises through
the other or "upcast" pit back to the surface. In a heavily-watered mine
it is often necessary to establish a special engine-pit, with pumps
permanently fixed, or a division of one of the pits may be devoted to
this purpose. The pumps, placed close to the point where the water
accumulates, may be worked by an engine on the surface by means of heavy
reciprocating rods which pass down the shaft, or by