MINING. © 121 
covered with asemicircular arch. The sole, where it consists of sand, and there 
is not an excess of water, stands very well. When it is very wet it becomes 
necessary to turn a reversed arch, upon which the walls of the gallery are 
built. Only one iron centring is used while constructing the arches, which 
is moved forward as the work proceeds. It is made of wrought-iron, and 
consists of three ribs which have the form of the outside of the walls (pl. 
25, jig. 18). These exterior centrings, if they may be so termed, consist of 
three parts, the bottom piece, and two similar half arches which leap over 
each other in the middle, and are fastened together by means of screws. 
The iron sole rests upon another of wood, and the three ribs of the centring are 
placed pearly two feet apart. Upon these ribs lie about forty iron plates, 
seven to eight feet long, half an inch thick, and four inches wide. By this 
means the walls and roof of the excavation are secured for a distance of 
seven feet, and when the arch is completed the supporting frame is moved 
on to support the next stretch. 
Most of the machine-chambers below ground consist of rooms for the 
water-wheels which are connected with the pumps, the steam-engines being 
very seldom beneath the surface. These chambers require to be walled up, 
partially or entirely, and great care is necessary in their location, as serious 
accidents may occur, entailing costly repairs. In pl. 24, jigs. 3, 4, and 5, is 
seen a wheel-house as usually walled up in the Saxon mines, the wheel being 
indicated in jig. 3 by the dotted circle. The water is admitted through 
suitable openings in the roof if the wheels be over-shot, or through the side 
walls of the chamber if under-shot wheels be used. 
5. SINKING oF SHAFTS. 
When a shaft is to be sunk into solid rock it is done by blasting. In 
this work great inconvenience is caused to the workmen by the water which 
issues from the cliffs in the rock and falls down upon them. In order to 
prevent this a gutter is cut in a spiral form along the sides of the shaft (pl. 
25, fig. 22), emptying into a small excavation in the solid rock, calculated to 
contain the water discharged in twenty-four hours, from which the water is 
raised in buckets when it is full. 
_ In England, especially in the vicinity of Newcastle, the shafts are made 
circular, the smallest being ten feet in diameter. The circular form is well 
adapted to shatts in strata of a small dip, and is also much used in the 
Liége coal-mines, and in some parts of France. For shafts in rocks of great 
dip, the rectangular form is preferable, particularly when they are to be tim- 
bered ; in Germany therefore the rectangular form is generally used, and the 
masonry is executed in four arches abutting against each other in the corners. 
1. Toveertne or Suarrs. The timbering of the shafts is the wood- 
work necessary to support the sides. In shafts which are to serve for 
a short season only, a temporary timbering is made use of, constructed 
in the following manner. Green oak, birch, or beech, is bent into hoops, 
which are placed one beneath the other as the shaft proceeds, and serve 
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