CRANE. | | Ventilation. | 331 
culty is experienced in freeing them from any excess of water, 
except in those cases where an extra large amount is un- 
avoidable, due to a return, through fissured ground, of water 
expelled or an inflow from other workings, and possibly the 
occurrence of ‘‘vadose’’ or underground waters under slight 
pressure. 
VENTILATION. 
The question of ventilating the mines in this district is 
comparatively simple: First, as there are no noxious and 
offensive gases associated with the formation ; second, as the 
workings are shallow. 
As arule, there is only one opening to amine. Occasion- 
ally there are two, where two mines on adjoining properties 
have been worked together and one large cavity results. 
VWigure 88. In such a case currents of air are set up in one 
direction or another, and one shaft becomes by natural selec- 
tion the upcast and the other the downcast. This selection 
of the direction of the air current is partially governed by 
prevailing conditions, such as a wet and dry shaft, the mouth 
of one shaft being more elevated than the other, etc. The 
air currents are easily regulated and directed ; for instance, a 
wet shaft may cool the air to such an extent that a down 
current results; an elevated shaft may so cool the air that 
in warm weather a down current is set up, while in cool 
weather, the air in the shaft, being warmer than that outside, 
rises. This reversal of air current may take place at night 
and in the morning, making two complete reversals for the 
day, but is more marked in the change from summer to win- 
ter and from winter tosummer. ‘The cut shows the direc- 
tion of the current in winter. 
22—vili 
