-bearing ground is
by compressed air. The shaft is lined with a cylinder of wrought iron,
within which a tubular chamber, provided with doors above and below,
known as an air-lock, is fitted by a telescopic joint, which is tightly
packed so as to close the top of the shaft air-tight. Air is then forced
into the inclosed space by means of a compressing engine, until the
pressure is sufficient to oppose the flow of water into the excavation,
and to drive out any that may collect in the bottom of the shaft through
a pipe which is carried through the air-sluice to the surface. The
miners work in the bottom in the same manner as divers in an ordinary
diving-bell. Access to the surface is obtained through the double doors
of the air-sluice, the pressure being reduced to that of the external
atmosphere when it is desired to open the upper door, and increased to
that of the working space below when it is intended to communicate with
the sinkers, or to raise the stuff broken in the bottom. This method has
been adopted in various sinkings on the continent of Europe.

Shaft boring.

The third method of sinking through water-bearing strata is that of
boring, adopted by Messrs Kind & Chaudron in Belgium and Germany. For
this purpose a horizontal bar armed with vertical cutting chisels is
used, which cuts out the whole section of the shaft simultaneously. In
the first instance, a smaller cutting frame is used, boring a hole from
3 to 5 ft. in diameter, which is kept some 50 or 60 ft. in advance, so
as to receive the detritus, which is removed by a shell pump of large
size. The large trepan or cutter weighs about 16 tons, and cuts a hole
of from 9 to 15 ft. in diameter. The water-tight lining may be either a
wrought iron tube, which is pressed down by jack screws as the borehole
advances, or cast iron tubbing put together in short complete rings, in
contradistinction to the old plan of building them up of segments. The
tubbing, which is considerably less in diameter than the borehole, is
suspended by rods from the surface until a bed suitable for a foundation
is reached, upon which a sliding length of tube, known as the moss box,
bearing a shoulder, which is filled with dried moss, is placed. The
whole weight of the tubbing is made to bear on the moss, which squeezes
outwards, forming a completely water-tight joint. The interval between
the back of the tubbing and the sides of the borehole is then filled up
with concrete, which o