116 TECHNOLOGY. 
rock in a wide drift costs far less than in a narrow one, not only on account 
of the greater facility of working by steps, but because the extent of space 
lightens the work. In soft but tolerably compact rock the levels are wrought 
by means of pick and wedges without blasting. The rock stands long enough 
to admit of carpentry or masonry being subsequently constructed, to prevent 
future falling of the walls and roof. 7 
1. Toweerine oF THE Levers. In driving a level which must afterwards 
be timbered, care must be taken that the requisite space within the timbers 
is secured. Timber-work is almost always cheaper than masonry, but wood 
rots and gives away under the pressure of the rock, and therefore requires 
to be replaced from time to time. In works upon a mine which is to be 
used only two or three years, timbering is used ; while in large drifts or adits, 
which serve for draining the mine, and therefore require to last for a con- 
siderable time, masonry is adopted. 
Before considering the details of timbering, we may make the following 
preliminary observations: First, we must observe the degree of compact- 
ness of the rock, and determine as far as possible the direction of the 
pressure. If, for example, the rock is split through in many places, and 
consists of broken and loose masses of stone, this is a sign that the mass 
exerts strong pressure, and must be supported by timbering. As the 
pressure of the rock is not always vertical, but lateral, the resistance fur- 
nished must correspond with the pressure. As the pressure which the mass 
exerts upon several points is less than when the whole mass has to rest on 
a single point, we must seek to bring the pressure to bear upon as many 
points as possible. 
The ends of every cross-beam used in a mine should lie in the rock. Two 
mortises or hollows must, therefore, be cut in the rock in which the ends 
of the cross-beams should he. These mortises must be 8-12 or 24 inches 
deep, according to the character of the rock. 
The cross-timbers are round, hewed, or split pieces of timber, which are 
laid in a horizontal position withia the level, parallel to each other, and at 
such distances from each other that from Sire to five may occupy a length 
of six yards. The timber-work of the levels further consists of double and 
single upright posts, standing under the cross-beams. In mining operations 
it often happens that the roof and sides of the level when it is first exca- 
vated are perfectly strong, but in the course of a few years large masses 
give way both in the roof and sides. Whenever there is any apprehension 
of this difficulty, double upright posts are chosen for timbering. If weak- 
ness in the roof alone is apprehended, the single uprights are used. The 
double upright posts stand perpendicularly to the bottom of the level, and 
are connected at the top by acap or cross-beam. A single upright stands alone 
under the cap. At Freiberg, in Saxony, where the lodes are not thick and the 
levels, consequently, are narrow, all the uprights are placed vertically ; but 
where the lodes are thick and the levels widen, the uprights or posts are 
placed wider apart at the bottom than at the top ( pl. 25, jig. 11a). This is 
made necessary by the width of the levels and the pressure. The uprights 
are set up slanting wherever the side pressure is greater than the pressure 
696 
