HEBBUBN COLLIERY. 359 



the mechanical troubles are broken cutter chains and trolley 

 chains. Most of the breakages are due to carelessness on the 

 part of the men. 



As chain breast machines require 12ft. space to work in, 

 when the roof is tender, the coal has to be undercut by hand. 

 If not too bad, the roof may be supported by 12ft. slabs resting 

 on two posts. 



Two single pole joint boxes are used at the entrance to 

 each going bord for connecting the trailing cable to the main. 

 Each pole having its own box, there is no possible chance of 

 accidental short circuiting. By suitable mechanism, the hole 

 through which the plug enters is closed by a door which can- 

 not open while the switch is closed. The main cable is car- 

 ried along the roof of the heading on the bord side, so that the 

 trailing cable shall not have to cross the rails and run the 

 risk of being damaged by passing skips. 



To save a long detour of half a mile or a mile for the cables 

 from the workings of the upper split to those of the lower 

 split, a borehole, 2jkin. in diameter, was put down by hand for 

 16ft. It was lined with a 2in.' tube, which was cemented in ; 

 the cable was passed down this and fixed with bitumen. 



The trailing cable is a twin, sometimes round in section 

 and covered with plaited greeiihide as a protection ; and some- 

 times it is flat in section, the cables being laid alongside each 

 other instead of being worked in with strands, as in the case of 

 the round cable. The flat cables do not coil up so well as the 

 round. They are run through rubber hose for protection. 



The old electric plant consisted of a McEwan engine which 

 drove a Jeffrey direct current generator for low tension. There 

 are equalising switches to equalise the power of the main and 

 spare generators. The recent installment consists of two Er- 

 nest Scott and Mountain generators, each of 100k. w., driven 

 by 14 ropes from a Eobey and Co.'s compound engine, 18in. 

 and 28in. diameter cylinders, and 3ft. stroke. All the shaft 

 bearings for the dynamos are on the ball and socket principle, 

 which allows for a certain amount of give and take, and ad- 

 justs any irregularities in alignment. There are two slip rings 

 for self lubricating purposes in each bearing, and there is a 

 sufficient supply of oil to last the bearing for three months. 

 There is a flexible coupling between the rope pulley and each 

 motor. Both halves of the coupling have 6 bolts arranged in 

 a circle, but the circles are of different radii. A belt is worked 

 in and out between the bolts of the two halves, and the whole 

 covered with a steel shell. The coupling can be readily dis- 

 connected by slipping the shell on one side and pushing the 

 belt off. All the brushes on the dynamo can be adjusted simul- 

 taneously by a hand screw, or each brush can be adjusted 



