COAL AND COAL-MINING.

7

over the furnace System, and the useful effect of a good fan seemsto be from 40 to 60 per cent. of the power employed. The quan-tity of air discharged varies with the size of the fan and the speedof rotation. In some of the centrifugal fans of about 16 feetdiameter, the quantlty of air in cubic feet per minute passedamounts to from 40,000 to 50,000, and in larger fans of 30 to 50feet diameter, 100,000 to 200,000 cubic feet per minute may bedischarged. In the Schiele fan the speed is very high, 150 to 300revolutions per minute, the diameter being smaller than some ofthe other forms, such as the Guibal, which is generally of a largerdiameter with a less velocity of rotation, say about 90 revolutionsper minute. An engraving of the Guibal fan, which is now largelyused, is shown in Fig. 1.

As a general rule no mine should have a ventilating power ofless than 100 cubic feet per minute for each man and boy employedin the Underground passages, and in mines making large quantitiesof fire-damp a Ventilation equal to from 200 to 600 cubic feet perminute per man should be attained.

The common methods of working coal in this country are “ long-wall ” and “stall and pillars,” with a modification of the latter called“ rances.” By the “ long-wall ” System all the coal is excavated,and it is the most profitable way of working. Before starting anycoal “ long-wall,” however, there are several circumstances to beconsidered, such as the nature of the roof, the property that mightbe injured by the subsidence of the superincumbent strata, and soon. In the “ stall and pillar ” System there is a great sacrifice ofcoal, generally about one-third, but often nearly one-half; this plan,therefore, should never be adopted if the coal can be worked “ long-wall.” Pillars are often left large or worked in “ rances,” with theIntention of being afterwards removed; . but this plan does notalways succeed. Large as the pillars may be, they often sink intothe pavement if it is soft, and cause a “creep,” which shatters thecoal, besides forcing the soft pavement up to the roof in the roads androoms or stalls, and the contemplated removal of the pillars is thusfrustrated. The edge seams of coal are worked “ long-wall ” in somecases, and “stall and pillar” in others. Instead of the pits beingsunk straight down, inclined shafts are driven through the bed ofcoal, with rooms branching off from either side of the incline, andto work these the men stand on the coal as a floor, having the coalalso as the roof. In the shaft, instead of a cage and slides, there is