21 
one case near 200,000 cubic feet of air in one minute are circu- 
lated through the devious passages, and rush to the upcast shaft 
with the velocity of a hurricane. 
Nearly related to this division, as regards the question of 
humanity, is the true construction and the preservation of mining- 
plans. Take an instance in which the loss of 100 lives may 
ensue from the ignorance of a physical fact. Those familiar 
with the mining districts are well aware that the great majority 
of their maps are laid down without any reference to the pheno- 
menon of magnetic variation. If, then, a colliery be in operation 
on the dip of old workings filled with water, the tapping of which 
would be death to all employed in the pit, and the maps had been 
constructed some years previously with respect to the magnetic 
meridian alone, the variation may in the mean while have so 
far changed that the exploring drifts supposed by the manager 
to be going clear of the known danger may, in reality, lead him 
straight to destruction. The art of surveying is, however, too 
important and extensive to be included in the lectures on 
Mining, and will ultimately, we hope, form the subject of a 
separate course. 
The last group of operations to be included is the dressing of 
ores, on the efficient conduct of which the success of many a mine 
may depend. Whilst the Schemnitz miner is able to work 
actual gold ores broken from great depths, which, besides a little 
lead, contain no more than one part of gold in 228,000 of stone,* 
and whilst the Russians wash in their stream-works sands con- 
tainining only one partin half a million, we learn from description 
that the Californian and Australian are employing processes 
more rude than what they might have copied from the miners 
of three or four centuries since, and that (inasmuch as the poorer 
* In 1841-2, when I passed some months among the mines of Hungary, 
much had been done and was still doing by my friends the late Oberst- 
kammergraf von Svaiczer and Mr. Riltinger, the Inspector of Stamp- 
works, for the improvement of the dressing of gold and silver ores ; and 
the works at Antal and Illia, near Schemnitz, were well worthy of ad- 
miration for their scale and economy. 
