1879.] Influence of Coal-dust in Colliery Explosions. 417 



Fig. 3. 



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standing the apparatus and the experiment. In all the figures, A repre- 

 sents the cavity in the roof : it is a galvanized sheet-iron cylinder, 

 4 feet long by 15 inches in diameter, covered at the top and open at 

 the bottom. There is a stuffing-box in its cover which allows a thin 

 spindle to pass through it in an air-tight manner. At its lower end 

 the spindle carries a fan which consists of a thin metallic disk 

 11 inches in diameter, having a hole 4 inches in diameter in its 

 centre, and with radial blades 1-J inches high on its upper surface. 

 When the fan revolves, the blades, which are nearly touching 

 the cover of the cylinder, throw out the air centrifugally and draw in 

 new supplies through the hole in the disk. Immediately below the 



