MINING. 123 
_ Ata depth of 216 feet a third tubing became necessary for a distance of 
24 feet, and at a still greater depth a fourth was required. 7. 24, fig. 19, 
is a vertical section of a portion of this dam. Above these cast-iron cylin- 
ders the shaft was lined with stone masonry. 
There is another species of damming, in which, instead of lining the gallery 
or shaft, the point from which the spring has burst is plugged up as it 
were. This occurs most frequently in a gallery, the whole of which, in 
such cases, is often closed by the dam. A bed of moss is first laid upon 
the sole, and the timbers of the dam are then built in, wedged, and caulked 
with moss. PJ. 24, jig. 28, shows an instrument used for enlarging the 
openings between the timbers, and jig. 29 the chisel for driving in the 
moss ; jigs. 26 and 27 are sections of such a dam, propped upon the front 
side to prevent bending; jigs. 22 and 28 are instances of the same in vertical 
shafts. | 
Sometimes the wall of the shaft is built upon an iron shoe, sharp at the 
bottom, and the excavation is made upon the interior, the shoe cutting its way 
down as the work proceeds, and sinking gradually with the wall. Between 
the wall and the sides of the shaft are scantlings, placed vertically to prevent 
interference between the masonry and the sides of the cut (pl. 24, fig. 20). 
At other times the sides of the shaft are supported at the time it is sunk by 
a temporary timbering of scantling (pl. 25, jig. 19), which gives place where 
the shaft is entirely excavated to the masonry walling. 
2. SHarr Masonry. When shafts are to be kept open more than six or 
seven years, masonry is preferred to timbering. The masonry is either 
laid in common or hydraulic cement, or is carried up dry where the ground 
is free from water. When oneor more sides of a shaft are to be secured by 
masonry, arches are sprung over the level below the shaft (pl. 24, fig. 13), 
and on these arches the masonry is carried up, presenting either a straight 
face to the shaft (pl. 24, fig. 10), or the concavity of an arch (jig. 11) 
when the rock is rather loose, and exerts considerable pressure on the wall. 
The empty spaces behind the walls are packed with rocks. /%igs. 12 and 
14 are sections of a rectangular shaft, all four sides of which are secured by 
masonry; jig. 15 is the plan. In shafts of great depth a partition-wall is 
built, separating the ascent-shaft from the service-sbaft, and affording addi- 
tional security to the masonry of the long sides of the shaft. Wooden par- 
titions are also often made of boards nailed against cross-pieces fixed in the 
masonry ; the boards are tongued and grooved, and closely fitted, the 
division of the shaft into two spaces serving for ventilation. Jigs. 24 a, 
246, and 25 a, 25 b, show a wooden partition; at the lower end (jig. 24 5) 
it is inclined to the side of the shaft, in order to prevent the buckets from 
catching under it. The ladders stand in the smaller portion of the shaft on 
foot-boards (fig..256), which occur every 30 feet; each foot-board has a 
man-hole (fig. 25a), through which a man can pass freely. 
In inclined shafts the masonry of the short sides is made in the same 
raanner as in vertical] shafts. The upper side is secured by a flat arch, or 
according to the dip and pressure of the strata by arches of more or less 
rise, resting below on strong supporting-arches (pl. 24, jigs. 16 and 17), 
703 
