601 
the floor, which, in the course of time, would completely con- 
ceal the pillars so that they would not be able to get at them. 
In his colliery, they did not leave pillars of sixteen yards or 
sixteen inches ; they took all the coal as they proceeded. 
Mr. Brakenridge said these were necessary for keeping 
up the ventilation. 
Mr. Briggs admitted it might be a necessity on Mr. 
Brakenridge's principle, but it was not even desirable on the 
plan pursued in his (Mr. Briggs') colliery. He might add 
that he concurred with Mr. Brakenridge upon the evil of 
carrying the return air through the cupola fire. At his 
colliery such air was carried from the mine by means of a 
dumb drift above the cupola fire. 
Mr. Brooke observed that there were twenty doors in 
the diagram. Now he thought doors in mines most objec- 
tionable, as in cases of explosion they were blown down. 
Mr. Brakenridge admitted that when doors were on 
the side of the workings next to the up-cast pit, they would 
be blown down, but not on the other side, where these doors 
were all placed. 
Mr. Brooke said the force of the explosion had nothing 
whatever to do with the current of air. The force of the 
explosion was caused by the rush of air to fill the vacuum 
caused by the explosion itself ; and, as would be seen, that 
force or rush would be in all directions, irrespective of the 
current of air passing through the mine. 
Mr. Brakenridge remarked that the risk was not in 
having the doors, but in having them left open, 
Mr. Brooke quite concurred in that ; and it followed 
from it that the more doors the greater the risk of some 
being left open. 
Mr. Jebson said his great objection to the plan was, that 
he did not see how it could be carried out in practice. He 
could not see the utility of placing the up-cast shaft at the 
