NORTH BULLI COLLIERY. 171 



causes the underweight to bind, throws too much weight on 

 the face, and crushes the timbers. If the overweight settles 

 too slowly the progress of the underweight is retarded. The 

 distance between the timbers, and the nuinbe* of rows, will 

 depend on the conditions of the seam, roof, floor, depth of 

 cover, etc. 



The coal is undercut by Sullivan's longwall machines, 



Fig 105 Sullivan Long Wall Machine. 



of which there are four, driven by electricity : the motor takes 

 280 volts, 105 amp. and has 1225 revolutions per minute. This 

 machine is similar to the Sullivan electric chain machine, the 

 chief difference being that though the cutter-bar is swung into 

 line with the machine when loading into a truck to flit to 

 another place, while at work it is placed at right angles to 

 the body of the machine. (Tiff. 105.) The machine 

 advances as the cutting proceeds, along a chain, placed parallel 

 with the face, which is threaded through the machine and 

 jacked at the ends. A driving sprocket engages with this 

 chain, and so procures the forward motion, the machine sliding 

 along the floor on a sheet steel shoe without the intervention 

 of rails. This coal cutter uses about 15 b.h.p., and will cut 

 120 yards long, 5in. high, for a depth of 1| yards, in an eight- 

 hours' shift. It is worked by a driver, an assistant and a 

 boy. The boy throws out the cuttings from the side of 

 the machine, while the assistant cleans it away from under the 



