401 



lookers of collieries. To afford as good a ventilation of a 

 mine as the circumstances will admit, does not allow of a fire 

 of any size to be placed at the bottom, sometimes merely in 

 a fire pan, or in a grate of any size; but the dimensions of 

 the fire and fire place must be in proportion to the area of 

 the shaft through which the ascending current of air has to 

 rise, otherwise the fuel in the grate not being sufficient to 

 rarefy the whole portion of air in the flue, the rising current 

 of heated air is met by a descending current of cold air, and 

 the circulation is thereby impeded. This is a common defect 

 in the construction of house chimneys, and the smoke, met 

 by the descending current, is borne back into the room. 

 Tredgold's rule for the orifices of chimneys according to the 

 height and magnitude of the fire place is equally applicable 

 to the upcast shaft of collieries, viz., to multiply by 17 the 

 length of the fireplace in inches, and divide by the square 

 root of the height of the chimney (above the grate) in feet, 

 and the quotient is the area in inches for the aperture of the 

 chimney.* 



I know of no colliery where these conditions are fulfilled 

 with regard to giving full effect to the first force of ventila- 

 tion, for although it may be necessary to use the upcast shaft 

 for other purposes, still if one fire is not large enough, other 

 furnaces might be added. 



The mode in which the subterranean ventilation is effected 

 necessarily depends upon the plan on which the coal is ex- 



* Example For a grate 18 inches wide : — 



The chimney, if 36 feet high, should only have an aperture of 51 inches area, 

 i. e. about 7 inches square. 



18 X 17 = 306 ^ ( V^36 = 6) = 51 inches. 

 A fire at the bottom of a pit, having a fire-place 12 feet or 144 inches v^'ide: — 

 The chimney, 300 feet high (100 yards) requires an aperture of only 144 



inches, or 1 foot square. 

 Therefore two fires, as at Haswell, or three fires, would be required for shafts of 

 8 feet diameter, and 100 yards in depth. 



