bering, or more usually by arching in masonry or brick-work. Levels
like that in fig. 2, which are driven across the stratification, or
generally anywhere not in coal, are known as "stone drifts." The sinking
of colliery shafts, however, differs considerably from that of other
mines, owing to their generally large size, and the difficulties that
are often encountered from water during the sinking. The actual coal
measure strata, consisting mainly of shales and clays, are generally
impervious to water, but when strata of a permeable character are sunk
through, such as the magnesian limestone of the north of England, the
Permian sandstones of the central counties, or the chalk and greensand
in the north of France and Westphalia, special methods are required in
order to pass the water-bearing beds, and to protect the shaft and
workings from the influx of water subsequently. Of these methods one of
the chief is the plan of tubbing, or lining the excavation with an
impermeable casing of wood or iron, generally the latter, built up in
segments forming rings, which are piled upon each other throughout the
whole depth of the water-bearing strata. This method necessitates the
use of very considerable pumping power during the sinking, as the water
has to be kept down in order to allow the sinkers to reach a water-tight
stratum upon which the foundation of the tubbing can be placed. This
consists of a heavy cast iron ring, known as a wedging crib, or curb,
also fitted together in segments, which is lodged in a square-edged
groove cut for its reception, tightly caulked with moss, and wedged into
position. Upon this the tubbing is built up in segments, of which
usually from 10 to 12 are required for the entire circumference, the
edges being made perfectly true. The thickness varies according to the
pressure expected, but may be taken at from 3/4 to 1-1/2 in. The inner face
is smooth, but the back is strengthened with angle brackets at the
corners. A small hole is left in the centre of each segment, which is
kept open during the fitting to prevent undue pressure upon any one, but
is stopped as soon as the circle is completed. In the north of France
and Belgium wooden tubbings, built of polygonal rings, were at one time
in general use. The polygons adopted were of 20 or more sides
approximating to a circular form.

[Illustration: FIG. 2.--Shaft and Level.]

Pneumatic sinking.

The second principal method of sinking through water