436 
SALT. 
there are springs which seem to follow the varia- 
tions of the barometer, and become stronger or 
weaker, in proportion as that instrument rises or 
falls. Others diminish in very cold weather and in- 
crease in hot, without being sensibly affected either 
by a wet or dry season. 
Salt springs are very numerous, and occur in most 
parts of the world. Those of our own country, 
situated at Northwich, are well known for the 
great quantity of salt which is annually obtained 
from them. The springs are from twenty to forty 
yards below the surface of the earth, and the water 
is raised by the assistance of a steam-engine, and 
conveyed through long troughs to the brine-pits, 
where it is evaporated in large iron pans till the salt 
crystallizes. An immense quantity is collected in 
this way, no less than 45,000 tons being annually 
manufactured in the town of Northwich. 
We must now proceed to mention the rock salt, 
or sal gem ; which, for the immense rocks of it that 
occur in different countries, and the mines that are 
excavated for the purpose of obtaining this useful 
commodity, becomes an object highly deserving our 
attention. 
The name of sal gem has been given to this kind 
of salt on account of its more than ordinary hard- 
ness, and that it sometimes assumes the colour and 
almost the transparency of a precious stone. Beds 
of sal gem are found considerably below and as 
much above the surface of the earth. At Wielitska 
in Poland we descend for this mineral to the depth 
