77 



be incurred in the use of substitutes such as steel or cement. In most 

 instances the recovery of such supports is dangerous and impracticable. 



The forms in which this wood is needed are props, legs and bars, 

 lagging, caps, mine ties, and lumber. 



Props are small timbers, either split or round, used in temporary 

 openings, such as in rooms for supporting the roof. Props run from 

 3 to 5 inches in diameter at the tip and from 2J4 to 10 feet in length, 

 depending upon the thickness of the coal seam. 



The vertical pieces used in the entries and other more permanent 

 positions are called legs or posts and the horizontal piece is called a 

 cross-bar or, more briefly, a bar. These may be (! or 8 inches in diam- 

 eter at the top end and up to IG feet long. 



Lagging, or riprap, consists of small poles or boards which extend 

 from one set of legs and bars to another to keep small pieces of roof 

 or wall from falling into the entries. In most cases the lagging is used 

 only where the ground is loose, as the rooms are usually cut in the 

 coal so that the walls stand well without support. The quantity of 

 this material used in Illinois coal mines is comparatively small. 



Caps are pieces of board about one foot square used to place on 

 top of props or legs for the purpose of tightening up the pieces which 

 support the roof. Old props might be cut up and used for this same 

 purpose. The ordinary size for cap pieces is 12x13 inches and 1 inch in 

 thickness. Mine ties may be as small as 3J^ feet long by 2x4 inches, 

 and up to 5^ feet long by 5x6 inches. Lumber is used for general 

 construction, and for mine cars. 



The production of bituminous coal for Illinois in 1921, of 80,121,948 

 tons is taken as the basis for determining the quantity of wood required 

 in mining this coal. Of this total, 78,399,082 tons were the output of 

 shipping mines. 



The total consumption of wood in coal-mining was obtained for 

 mines producing a total of 22,146,784 tons, or 27.61: per cent of the 

 total output, well distributed over the entire state. The remainder was 

 calculated by proportion, assuming the same rate of consumption. On 

 this basis the coal industry consumed in 1921, 21,552,250 cubic feet of 

 wood in all forms, of which 19,710,000 cubic feet went into the mines 

 in the form of timbers* and the remainder, 1,842,260 cubic feet, was 



• All forms of wood were converted into cubic feet by proper equivalent. For 

 props, the truncated cone formula was u.sed: 



V = 0.2618 L (D" -I- dD + d«) 



144 



whore V = volume. L = length, D = top diameter, and d = butt diameter. 



B'or lumbir G board fi et wan taken as equaling 1 cubic foot. 



An average taken on 3.300,000 pieces indicated that the dimensions of the aver- 

 age mine prop were 5.2 feet lontr by 5.4 Inches at the middle, or a little over 6 inches 

 at the small end, and containod .83 cubic feet. Mine ties averaged .7 cubic feet. 

 Mine caps are of various sizes, but averaged 1 board foot or % cubic foot. L>eg8 

 and bars averaged 1.91 cubic feet. 



