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mode of vanquishing the difficulty was to dam back the water, 
to employ a constant resisting force which might be maintained 
at small expense, in place of a moving power of enormous cost. 
It was, in fact, to pump into the iron cylinder which formed his 
shaft such an amount of air that the pressure on the bottom 
from within should be equal to the pressure from without ; and 
the water was thus prevented from rising above a given height. 
Placing a cover on the cylinder, through which two pipes are 
inserted, one conveying the compressed air into it, the other 
dipping into the watery stratum, he found a stream of water and 
sand poured through the latter whenever it was unable to escape 
rapidly enough elsewhere. Then, in order that the men might 
enter upon or leave their working place without disturbing the 
equilibrium of the forces, he applied the principle of the canal 
lock, fitting an upper chamber in his shaft, where, when the 
upper door was closed and the lower one opened, all was filled 
with the compressed air ; when the lower one was closed and 
the upper opened, the air-lock was relieved from its super- 
abundant aii-, and men or materials might be introduced. Pages 
would be filled with the details of the difficulties met and over- 
come, and of the successful adoption of the principle in the sink- 
ing of larger and deeper shafts ; suffice it to say, that M. Triger 
succeeded, surrounded by water, in joining his cylinder to the 
solid rock at a depth of 82 feet from the surface, having proved 
that human life could be supported, and work done, for many 
hours together under a pressure of atmospheres. His pro- 
cedure is marked by manifold advantages, and admits of various 
applications ; — witness the removal of rocks in the harbour of 
Croisic, on dry ground, whatever the state of the tide. Verily, 
if Canute had possessed a Triger among his courtiers, he might, 
to better purpose, have defied the rising flood to touch his 
royal foot! 
We shall bo unable here to glance even rapidly over the 
many systems devised for working out the minerals attained by 
the foregoing operations : let us only scan an isolated case. 
The magnificent seam called the “ thick coal ” of Dudley has 
O •/ 
been worked throughout the entire field on a principle which 
by taking the whole height, 30 feet, at one time, has been 
6. b 2 
