MINING FOR COAL 



327 



must be left, to prevent the pillars sinking into the pavement, and producing a 

 creep. 



' 3. If the coal is very soft, or has numerous open backs and cutters, the pillars 

 must be left of an extra size, otherwise the pressure of the superincumbent strata will 

 make the pillars fly or break off at the backs and cutters, the result of which would 

 be a total destruction of the pillars, termed a crush or sit, in which the roof sinks to 

 the pavement, and closes up the work. 



' 4. If the roof is very bad, and of a soft texture, pillars of an extra size are re- 

 quired, and the rooms or boards comparatively very narrow. 



' In short, keeping in view all the circumstances, it may be stated generally, that 

 when the coal, pavement, and roof, are good, any of the systems before mentioned 

 may be pursued in the working ; but if they are soft, the plan is to work with rooms 

 of a moderate width, and with pillars of great extra strength, by which the greater 

 part of the coal may be got out at the last of the work, when the miners retreat to the 

 pit bottom, and there finish the workings of a pit.' f^^ i^s^sa 



Fig. 1504 represents the effect of pillars sinking into the pave- 

 ment, and producing a creep ; -And Jiff . 1505 exhibits large pillars 

 and a room, with the roof stratum bending down before it falls 

 at a. Thus the roads will be shut up, the air-courses destroyed, 

 and the whole economy of the mining operations deranged. 



In the ' Eeport from the Select Committee of the House of Lords, appointed to take 

 into consideration the state of the Coal Trade in the United Kingdom,' printed in 

 June 1829, under the head of Mr. Buddie's evidence, we have an excellent description 

 of the nature and progress of creeps, which wo have adverted to in the preceding ac- 

 count. The annexed figure (1506) exhibits the creep in all its progressive stages, 



1504 



1505 



1. First stage of active creep. 



2. Second do. 



3. Third do. 



4. Fourth do. 



5. The metal ridge closed, and the creep beginning 



to settle. 

 6 and 7. The creep settled, the metal ridges being 



closely compressed, and supporting the roof. 



from its commencement until it has completely closed all the workings, and crushed 

 the pillars of coal. The section of the figures supposes us standing on the level of 

 the different galleries which are opened in the seam. The black is the coal pillars 

 between each gallery ; when these are weakened too much, or, in other words, when 

 their bases become too narrow for the pavement below, by the pressure of the in- 

 cumbent strata, they sink down into the pavement, and the first appearance of creep 

 is a little curvature in the bottom of each gallery : that is, the first symptom obvious 

 to sight ; but it may generally be heard before it is seen. The next stage is when the 

 pavement begins to open with a crack longitudinally. The succeeding stage is when 

 that crack is completed, and it assumes the shape of a metal ridge. The next is 

 when the metal ridge reaches the roof. The following stage is when the peak of the 

 metal ridge becomes flattened by pressure, and forced into a horizontal direction, 

 and becomes quite close ; just at this moment the coal pillars begin to sustain part of 

 the pressure. The next is when the coal pillars take part of the pressure. The last 

 stage is when it is dead and settled , that is, when the rnetul or factitious ridge, formed 

 by the sinking of the pillar into the pavement, bears, in common with the pillars of 

 coal on each side, the full pressure, and the coal becomes crushed or cracked, and can 

 be no longer worked, except by a very expensive and dangerous process. 



The proportion of coal worked out, to that left in the pillars, when all the coal 

 intended to be removed is taken out at the first working, varies from four-fifths to 

 two-thirds ; but, as the loss of even one-third of the whole area of coal is far too much, 

 the better mode of working, suggested in the third system, ought to be adopted. 



The proportion of a winning to be worked may be thus calculated : Let Jig. 1507 

 be a small portion of the pillars, rooms, and thirlings, formed in a coal-field ; a, a, 

 are two rooms: b, the pillars; c, the thirlings (or area worked out). Suppose the 

 rooms to be 12 feet wide, the thirlings to be the same, and the pillars 12 feet on each 

 side ; adding the face of the pillar to the width of the room, the sum is 24 ; and also 

 the end of the pillar to the width of the thirling, the sum is likewise 24 ; then 

 24 x 24 = 576 ; and the area of the pillar is 12 x 12 = 144 ; and as 576 divided by 144 

 gives 4 for a quotient, the result is, that one-fourth of the coal is left in pillars, and 

 three-fourths extracted. Let d, e, f, g, be one winning, and g t e, k, h, another. By 

 inspecting the figure, we perceive the workings of a coal-field are resolved into quad- 

 rangular areas, having a pillar situated in one of the angles. 



