COAL MINING. 2 309 
less narrow work than obtains in the other systems of mining, and as 
narrow work always adds to the cost of coal mining, it is the cheapest 
way of working out a mine. As 75 per cent. of the coal is recovered 
in working forward, the pillars are abandoned when the rooms are all | 
worked out. Under this system the recovery of the pillars is im- 
practicable. 
The deeper pits are sunk, the greater the pressure upon the 
underground workings; hence, in the second system, which pre- 
vails in the deeper mining districts, the amount of pillar coal left 
under ordinary conditions is proportioned to the depth of the shaft. 
In the following table, in which the rooms are supposed to be fifteen 
feet wide, and the air-way six feet wide, the fincreasing thickness of 
pillars is proportioned to the depth of the mine: 
Feet of depth. Size of pillars, in feet. Proportion in 
pillars. 
iL AO Memencrc nea tessenciltessercciccccesstsecs's « GOKU One, Teeiee ose esinsese umecnecisneeetree sss 41 per cent. 
PAM ee see earc ddan eee canst cesiaticn oseaes § GORAIG aes. csi. canoe wviekls letilaedbaldaeteie dees HOS 
BOOM ecewsmecceciems eras neo Nhs Laaie es aes 2 GORI rertser. sos cet sists s saieets oiottvns slecuestsledelee 52 i 
ASO Ree side ae ence ct Rec sic siegief asi sles ects OO xe2 Ae Scthnecceisisonsens olaoscestesioeacasets 57 re 
GOD ance rceneae et eesb oseeneselc cess OO KAT SAR ieee) Cecsa es tdaks Potins siteoreseeatec 59 
LAO eescnabacscosa ci necnesmts Ss seSuisiaesesaen GOXBG TEs. cesta. acess stecas cosuseceeus cree sis 61 
SADR rae saicdcecnnes ecsces ssacoeoas so crineoes ((SXAZ Mavaccetees Suter eneececdons -tusiieecones ot 63 if 
SOOT sce eee Seisicocisnep awisjecienesels es USA Mecccasis oh enlddenennce neue tiger tcciscces 66 es 
LO SO eee Ee tae. cates ecoulsocusiae: WSXAS Li. cacecte duet uetiamastene ss tebe cal ces 69 “ 
ROOM cninas scans senedoteiieh ohoces saseees SAD Ie. Secs dnote cieee dete en ote es acsecle os 71 “f 
LB 20 Rei rwenele Maine, SOBs. IS GE ug SANA teeta ceed ce omnis stecidt aloe otis abi cs 73 ie 
DAA esse supine Capaie wees desis sadaetes SAGO once acces cates sess cceesss ales sceail ne 75 i 
LS GOR Ror err cnc eee eee cine ecnro es «5 DOGS 20 AGIs Seek sania Sando dateasseeas cs 77 *f 
LGSO Met eee Se ok Ae esterase coe se dette ns DOK ee eee Hoste Oo ks, SS, &. 78 nf 
L800 eee Bees idan. Couaanetiaaes ge tided swe QO REA escacteaelccsaacts <ouses 4p hers secep coteee se 79 f 
The above table is, of course, general rather than specific, for there 
may be other conditions which determine to a considerable extent the 
width of rooms and size of pillars, as, for example, the nature of the 
floor as to its hardness and softness as well as the depth of its softness, 
the nature of the coal as to texture and hardness, and the character of 
the face and butt slips of the mine; also, the nature of the roof of the 
mine, whether compact and firm, or loose and soft. 
After the rooms are all worked out by the system of leaving strong 
pillars, the pillars are attacked at the far end of the mine and worked 
back, the miners retreating under cover of the remaining pillars. When 
the excavated area begins to crush and fall down, part of the pillar coal 
