\ 820.] -Dr. M* Sweeny on the Ventilation of Mines. 377 



copper of the same breadth as the plate of zinc, the eflPect 

 differs from that which we have just mentioned only in being 

 much feebler. On the other side we increase the effect a httle 

 by making the conductor very short. Fig. 11 represents the 

 perpendicular section of this arrangement in the direction of the 

 breadth of the trough. Fig. 12 exhibits the same arrangement in 

 perspective. It is obvious that a c b d ef represents the con- 

 ducting plate, and c z zf the plate of zinc. In this arrangement 

 the north pole of the needle will be attracted towards the plane 

 oi a b c, and the south pole will be repelled from the same plane. 

 e df will have contrary effects. Here we have an apparatus 

 whose extremities act like the poles of the needle. But it must 

 be acknowledged that only the faces of the two extremities, and 

 not the intermediate parts, have this analogy. 



We may likewise make a moveable galvanic apparatus of two 

 plates, one of copper and one of zinc, twisted into a spiral, and 

 suspended in the fluid conductor. This apparatus is more move- 

 able ; but more precautions are necessary not to be deceived 

 when we make experiments with it. 



I have not yet found a method of making a galvanic apparatus 

 capable of directing itself towards the poles of the earth. For 

 this object it would be necessary to possess apparatus much 

 more moveable. 



Article X. 



Observations on the Ventilation of Mines. 

 By Joseph M'Sweeny, M.D. 



(To Dr. Thomson.) 



SIR, J^u/if 31, 1280. 



The safety lamp of Sir H. Davy has enabled miners to 

 descend into an explosive atmosphere with impunity 5 but when 

 we reflect that only a wire gauze liable to accident screens so 

 many from destruction, we must admit that every precaution 

 should be taken to prevent the accumulation of noxious gas. I 

 am not aware that water blowing engines* have been proposed 

 for ventilating mines. The water escaping from crevices of a 

 mine could be conducted by pipes to supply these engines, the 

 air extricated could be conveyed from them up the shaft by 

 tubes, and atmospheric air would descend to occupy its place. 

 In this way even carbonic acid gas may be got out. It is well 

 known that gas can be got out of a mine by a tube communicat- 

 ing with the under valve of a large bellows ; but if by the act of 

 getting out gas we could raise the water out of the mine, it 



• Vide Ferguson's Lectures by Brewster. 



