MINING. 119 
witz, temporary posts are erected on timbers placed in a wedge form, anda 
narrow opening, gradually enlarging, is carried forward in the ground to be 
worked (pl. 25, fig. 17). After the ground is drained permanent timber- 
work is constructed. 
2. Masonry or Levets. Masonry is always to be preferred to timbering 
if the adit or level to be driven is long, provided cheap and suitable mate- 
rials are at hand, and also if the mine has been opened several years. 
Masonry is indispensable where the adit or level is driven in very soft or 
marshy ground, where the object is not only to resist the pressure but to 
prevent the flow of water into the mine, and in all wide galleries or tunnels, 
such as are made on canals or railroads. All kinds of hard stone are used 
as materials for masonwork. When bricks are used they must be burnt very 
hard. The stone obtained in the mine is rarely suitable for the masonry of 
the adits or drifts. For dry masonry only rubble-stone and moss are used. 
For cementing the mason-work common lime and sand mortar or hydraulic 
mortar are used. 
There should be no empty space behind the masonry, or between it and 
the walls or roof of the drift. If this essential condition is secured, mason- 
work will receive a pressure on all parts of its exterior surface, and there- 
fore can only be destroyed by falling into the inside of the drift. In ground 
where there is no very strong pressure, and where at the same time the 
roof and the walls or sides of the drift are to be sustained, the masonry 
generally consists of a semicircular vault, or a right cylindrical arch resting 
on two piers extending perpendicularly along the walls of the drift. If the 
bottom of the drift is incapable of supporting the piers, the following means 
of obviating the difficulty are adopted. 1. The piers are placed on sills of 
oak wood, as in pl. 25, fig. 12. 2. Beneath this sill is constructed an 
inverted or reversed arch. 8. A perfectly closed elliptical arch is con- 
structed, the longer axis of which is vertical (jig. 14). The first method is 
adopted when the side pressure is weak. The second and third are resorted 
to when the drift is of large dimensions and the pressure is very consider- 
able. The lower curve of the ellipse may be flattened to prevent the 
gallery from being too high. 
We will now refer to some examples of the appropriate masonry of 
mines. In loose ground, like clay, sand, &c., and at a slight depth below 
the surface, where the excavation is afterwards to be built in with masonry, 
the walls and roof are temporarily supported by props and cross-beams. In 
this kind of ground, dry walls filled in with moss are often used, which are 
made 20-24 inches thick. These walls are better in marshy ground than 
those cemented with mortar. If the pressure is very considerable, and the 
ground at the same time marshy, hydraulic cement is used, and small canals 
are left here and there, by which the water may flow into the adit. 
If the proper curve for the arch of the masonry is fixed upon, centrings, 
constructed according to the condition and weight of the masonry, are 
placed for supporting the arches while being built. 
The most general rule for constructing arches in the adits or drifts of 
mines is, thatthe chord of the are should be perpendicular to the direction 
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