On the Ventilation of Mines. 81 



In the heat of piimmer the current is likely to descend by the 

 shaft whose top stands highest, and to make its exit by the shaft 

 whose top stands lowest, in the cold of winter this current w il! 

 be reversed. When tlie temperature is equal above and below, 

 and where the current is too feeble, it must be promoted by fires 

 or other artificial means. 



To direct this current through all parts of tlie workings, 

 various contiivanccs have long been used. These seem to be 

 all superseded bv Mr. R van's plan dcscrilied in your last num- 

 ber, and the plan of Mr. Menzics, which is somewhat similar. 



The immediate object of both these plans is to obtain from 

 all parts of the interior of the mine an uninterrupted ascent of 

 the roof, to the upcast pit, through which the current ascends. 

 If such an ascent can be obtained, the explosive gas will cer- 

 tainly be discharged by its own specific levity. But such an 

 ascent in the roof of a mine is sometimes jireventcd or destroyed 

 by depressions in the roof, and Ijy laige masses of the roof which 

 fall, leaving cavities to be receptacles for large quantities of ex- 

 plosive gas. 



Besides, these plans, at least that of Mr. Menzies, is not so 

 well calculated to ensure the discharge of the carbonic acid gas, 

 which is itself a great object in ventilation. This gas is spe- 

 cifically heavier than common air, and lies in the most depressed 

 parts of the mine. 



Inequalities in the roof or floor of the mine provide for the 

 retention of one or other of these gases, even in spite of a cur- 

 rent of air passing through tlie mine. This may be illustrated 

 by a transparent arched tube, transmitting a current of coloured 

 fluid or of air. Let an air bul'ble, or bead of air, occupy the 

 highest part of the arch, while a stream of coloured fluid is forced 

 through the tube. Tins air-bubble will be seen keeping its 

 place, or, if pushed out of it, regaining it speedily. Again, let 

 the arch be inverted, and let a bead of coloured fluid occupy its 

 lowest part ; while a stream of air is forced througii the tube ; 

 the coloured fluid will always sj)eedily regain its situation in the 

 lowest part of the arch. In this illustration, the stream of fluid 

 in the one case, and the stream of air in the other, represent the 

 current of air ventilating the mine, while the bead of air in the 

 highest part of the arcli, and the bead of coloured fluid in the 

 lowest part, represent masses of the two foul gases retaining their 

 situation in cavities in the roof and floor of the mine. It is true 

 that a forcible current of air will dislodge these ))()rtions of foul 

 gas ; but the ventilating current is often not sufficiently powerful 

 for this purpose. 



But though this ventilation were complete u/icn once ata- 

 lUihc'dfihc danger from dclctcrioiis gases nuifit be guarded against 



during 



