248 L UPTON : GOLD, SLATE, AND SALT MINES IN GREAT BRITAIN. 
tons of salt being produced by burning one ton of coal. By con- 
tinually pumping the brine the salt pillars have been dissolved, and 
the roof of the mine has fallen down ten yards, and great holes have 
been made in the surface ; in Northwich the subsidence of the sur- 
face has caused many picturesque arrangements of an architectural 
kind, and in some of the houses the varying inclination of floors 
makes the visitor think he is at sea. All the new houses are now 
built with wooden frames filled in with brick. The inhabitants who 
depend for their living on tlie salt industry do not make much fuss 
about the subsidence of the surface here and there, for, paradoxical 
as it may sound, it is strictly true that their commercial prosperity is 
founded upon the abstraction of their foundations. 
Rock salt is now got from the lowest eight yards of the salt beds, 
which is the purest ; the shafts are 110 yards deep. The salt is got 
in one mine (the Marston mine) over an area of 40 acres, the roof 
being supported by pillars 12 yards square and 25 yards apart, thus 
the pillars left are only one-tenth of the total area, (fig. 7 .) And yet 
the whole 40 acres rests upon these pillars without any sign of 
cracking, proving at once the hardness of the roof and floor and of 
the pillars, every square inch of which must bear a pressure of 
3,000 lbs. It may, I think, be said that there is no example of a 
coal-mine standing open on pillars, which are only equal to one-tenth 
of the total area. 
The salt is got by cutting a leading stall or breast about 7 feet 
high next the roof ; the floor is then got by blasting as required; 
the 7 feet wall is got by holing or under-cutting, and blasting it 
down ; the blasting is done with gunpowder, and straws filled with 
powder for fuses : the holing is done with a pick ; but in one mine, 
at least, it is done by cutting-machines driven by compressed-air, which 
cut a nick 2 feet 6 inches under the salt. One of these machines 
will hole twenty-five yards in a shift of eight hours ; there are three 
machines in this mine. When compressed-air machines are used there 
is ventilation caused by the exhaust air ; where there are no machines 
there is no method of ventilation. There are two shafts, one up- 
cast and one downcast, but there is no separation between them, and 
they are close together. It is surprising that with blasting going on 
