LUPTON : GOLD; SLATE, AND SALT MINES IN GREAT BRITAIN. 245 
the breast advances it is not easy to repeat the examination of a roof 
10, 20, 30, or 40 )^ards over-head. The slate-rock, however, is very 
sound and safe, and it very rarely happens that any pieces fall. One 
manager has contrived an apparatus for sounding the roof, consisting 
of a small hammer, fixed on a pin at the end of a long pole or gas- 
pipe, and by means of a rod the hammer is made to hit the roof and 
so test its security by the sound. The mines are lighted with 
candles, which are the best kind of light yet invented for the use of 
the rock men and miners, as they can be placed exactly where 
required. 
The slate is got by blasting and wedging. It must be understood 
that to get slate as in getting any other solid substance it is neces- 
sary to have three lines of fracture before a single piece can be sepa- 
rated from the mass. 
In getting a lump of coal the three lines of fracture are : 
(1) The line of stratification where the coal joins the roof 
(2) The line artificially made by cutting through the fast end. 
(3) The line of cleavage called the face. 
By these three lines of fracture, rectangular blocks of coal are got. 
In a slate mine all tliree lines of fracture are natural, and sepa- 
ration takes place upon the application of pressure. These lines are 
as follows : figs. 3 and 4. 
(1) The cleavage or split of the slate. At Ffestiniog the plane 
of cleavage dips at an angle of about 12'' towards the plane of stratifi- 
cation. 
(2) The " pillaring" fracture which is a plane of cleavage 
vertical to the split, but in the same direction. 
(3) The joints which are irregular planes of natural fractures 
traversing the slate rocks at right angles to, and vertical to, the 
plane of cleavage. 
In working the slate the sides of the breasts or pillars are made 
parallel to the plane of " pillaring" fracture. A loose end once 
made, fresh blocks of slate are separated by blasting ; the drill-hole 
is about vertical to the cleavage, and causes the slate to crack along 
the pillaring line through the centre of the drill-hole, the length of 
this fracture being from joint to joint, and the depth equal to the 
