120 TECHNOLOGY. 
of the pressure. If the bottom and sides of the drift are capable of sustain- 
ing pressure, but the rock above or in the roof exerts a perpendicular pres- 
sure, an obtuse or surbased arch is employed. If parts of the walls of the 
drift are cracked and loose, the abutments of the arch should be laid deeper 
into the rock or ground on the sides of the drift, or so far that a solid point 
is found for them to rest on. If the sides of the drift are so broken that 
no solid support can be found for the abutments, the span of the arch may 
rest on the firm bottom of the drift, or strong slabs of stone. The section 
of the arch then forms an ellipse compounded of many arcs of a circle. If 
the sole or bottom of the drift is so yielding that no firm ground can be 
found even by excavations, then ground or foundation arches must be con- 
structed. These are flattened arches the chords of which lie in the direc- 
tion of the drift, the impost of each arch being on points of the sole which 
have been ascertained to be perfectly firm (pl. 25, fig. 20a6). Upon these 
foundation arches the side walls are constructed. Various means of remedy- 
ing the want of firmness of the sole of the drift may be resorted to. The 
sole may be covered with large slabs of stone, or an inverted or counter- 
arch may be constructed, and on its springings the side walls may be erected. 
If the sole is wholly excavated and peculiar strength is to be given to the 
gallery, sustaining arches may be thrown across the drift under the bottom 
of the gallery which is to be constructed. The arches should be at a distance 
of from four to eight feet from each other (pl. 25, jig. 21a@6). Against the 
springings of the sustaining arches, and at right angles to them, the ground 
arches above described (fig. 20) should be constructed, and upon them the 
walls of the gallery or adit are built. Rubbish should be placed so as to fill 
up the space beneath the sustaining and ground arches. If, finally, there 
is nowhere any solid rock, and the bottom of the drift is wholly soft and 
yielding, continuous elliptical curves must be used to form the gallery (pl. 
25, jig. 14). 
Pl. 24, jig. 2, represents a mode of constructing the masonry of an adit 
where the foot-wall furnishes the only firm support. The foot-wall forms 
part of the bottom of the adit, and a partial ellipse of mason-work is made — 
to rest on steps or projections of the firm foot-wall. 
Recently whenever practicable, an entire or partial ellipse has been used 
in the masonry of levels and adits. One advantage in using the ellipse is 
that it may be constructed within timber-work and piling, which is first 
constructed when the ground is soft and compressible (pl. 24, jig. 1). 
In building the roof arch, centrings are necessary, as in masonry above 
ground, and the work is carried on in the same manner, except that the con- 
fined space makes it more difficult and tedious. The laying of the key- 
stones in such cases requires particular care and skill. After the completion 
of the section of an arch, such as is shown in pl. 24, jig. 6, the centring should 
remain several days before it is removed. 
A peculiar method has been adopted in the lower levels of the mines of 
Freienwalde in Prussia. Here an iron supporting-arch was made use of to 
preserve the roof and sides of the gallery while the arch was being con- 
structed, and to keep out the water. The walls of the galleries afe vertical, and 
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