MINE. 



337 



If the cover is of clay, with sand backs and oozings 

 of water, so that it will not stand firm for even a foot 

 or two, it may be sunk through, and secured by the 

 following process. 



For circular pits, circular fellies of wood, named 

 crib?, and for square or octagon pit?, square balks of 

 wood, named bars, are prepared, and used as A tempo- 

 rary cradling until the mason work is built. The width 

 of this carpentry depends upon, whether the timber is 

 to be drawn out as the building advances upwards, or 

 if it is to be allowed to remain, and the mason-work 

 built within it. The shape of the pit, where the tim- 

 ber is used, is either a circle, a square, or an octagon. 

 The temporary cribs are formed of oak, ash, or elm, 

 about seven inches in the bed, and five inches deep. 

 The balks are of fir, and from six inches to a foot 

 square, according to circumstances. The joints of the 

 circular cribs are plain, and bevelled to the radius of 

 the circle. The joints of the balks for a square or oc- 

 tagon pit are half checked. 



I f the timber is to be left in the pit, and the masonry 

 built within it, the quality of the wood need not be 

 much attended to ; but if it is to be drawn and used 

 gain, it requires to be good, in order to bear the fa- 

 tigue it is exposed to, without being rendered useless. 



With circular crib* of wood, .backing deals are used 

 of about 1 1 inches thick ; the pit is then sunk a* far as 

 the cover will admit with safety ; a strong crib is plac- 

 ed at the top or pit mouth, in a level position ; the 

 backing deals are placed all around, and reach to the 

 bottom of the space sunk ; cribs are then placed about 

 two feet apart, and the lower crib of the set is fixed 

 the half of its depth lower than the ends of the back- 

 ing deals ; the cribs are secured at the joints by thin 

 spars of wood overlapping them, and they are kept in 

 their horizontal position, either by brackets placed un- 

 der them, and nailed to the backing deals, or by a few 

 deals placed inside of the cribs, and nailed to each. Par- 

 ticular care is requisite, in sinking through the cover, 

 to have it no wider than necessary, in order that the 

 backing deals may bear against it, and if there are any 

 vacuities, these must be filled up with rubbish well 

 packed. When the first or upper space is thus secur- 

 ed, another space is sunk, of such a depth as the cover 

 will admit of; then backing deals and cribs are plac- 

 ed in the same manner as above described, the end 

 of the second set of deals having a hold of half of the 

 lower crib of the fint set. In UIM manner is the cover 

 sunk through and secured, until the rock head is found, 

 when the masonry is begun, and carried upwards to 

 the top ; if the cover is of such a kind as to admit the 

 cribs and backing deals to be drawn, these are taken 

 out in such lengths at a time as are judged safe, and 

 the space betwixt the masonry and cover filled up with 

 any kind of rubbish, firmly beat in. If there is any 

 risk of the cover slipping, then the carpentry, or the 

 greater part of it, must be allowed to remain in the 

 sides of the pit. 



In sinking square pits through a firm cover, the same 

 kind of process is adopted as that with the circular 

 cribs, the only difference being in the joints, which are 

 half checked. 



In sinking through a firm cover with a pit of an octa- 

 gon form, the bars or balks are half checked in the 

 ioints, and put in close to one another ; this is termed 

 being placed skin for skin. If the cover is of a very firm 

 contutence, their scantling or dimensions are small ; if 



i less firm, they are made proportionally strong. 

 The process carried on in sinking to the rock head is 

 VOL. xiv. PAHT i. 



the same as before described, the depth of cover passed 

 through and secured, each stage being regulated ac- 

 cording to the tenacity of the cover. The masonry is 



carried upwards from the rock head in the same man- 



.?. i >i i 

 ner as before described. 



It is a common practice to place two strong logs of 

 wood at the top of the pit, parallel to each other, and 

 at a distance, equal to the diameter of the pit, resting 

 on cross sills, laid upon the surface of the ground : 

 these are termed the hanging sets. From them the 

 cribs or bars are suspended, and prevented from slip- 

 ping downwards, by a few planks placed in a perpen- 

 dicular direction along the face of the bars, and to these 

 planks each bar is nailed. 



When the cover is of soft mud, which is a common 

 circumstance in many coal districts, it is passed through 

 cither by bars laid close to each other, or, if this mode 

 is not found practicable, the operation of tubbing is re- 

 sorted to. 



When bars are used, each set requires to be put into 

 its place as sogn as the pit is sunk the depth of a bar. 

 In this manner the sinking proceeds till the rock head 

 is found, when the masonry is begun, and completed 

 as before mentioned. It is necessary to remark, that 

 there is great attention required in this operation ; for 

 if the pit is sunk below the last placed bars, and not 

 immediately secured by additional bars, the mud will 

 well out into the area of the pit, and, when this begins, 

 it is almost impossible to stop it. The consequence of 

 which generally is, that a void is formed at the back of 

 one part of the bars, the resistance is then lessened, and 

 the pressure operating powerfully on the opposite side, 

 the bars give way, and the pit is frequently lost. It 

 is also found necessary, sometimes, before a bar can be 

 placed into one side, to drive a row of thin sheeting 

 piles, from three to four feet Ion/:, into the mud upon 

 the other sides in face of the ban, which prevent tin- 

 mud coming into the pit while the other bar is fixing ; 

 and when this bar is placed, one side of the sheeting 

 piles is removed and driven at the face of the last pla- 

 ced bar, till the adjoining bar is fixed. In this man- 

 ner the bars are placed progressively. 



When the mud is found to be so very soft that the 

 operation of tubbing must be resorted to, a circular tub 

 is formed of the requisite diameter. It is made of plank, 

 from two to three inches thick, with the joints hcvelUd 

 to the radius, inside of which are cribs of hard wood, 

 placed at from two to four feet asunder, according as 

 the circumstances of the case may require. The cribs 

 are made of the best heart of oak, sawn out of the na- 

 tural curve of the wood ; suitable to the radius, in seg- 

 ments of from four to six feet in length, from eight to 

 ten inches in the bed, and five or six inches in thick- 

 neas. The reed of the wood require to be clean, with- 

 out any twist or cross grain, for the greater strength 

 and security of the work. The length of the tub M 

 made from nine to twelve feet long, if the mud is of 

 that thickness ; but if the mud is very thick, a succes- 

 sion of tubs is placed one above another. The tub 

 first to be used has the lower end of the deals made 

 thin all around, and shod with sharp iron in the face. 

 If the pit is previously secured to a certain depth, then 

 the tub is constructed to pass within the cradling, and 

 lowered down with tackles till it rests among the 

 soft mud, it is then joaded with iron at the top, so that 

 it may sink as the mud is excavated. If the first tub 

 does not reach to the rock, then a second tub of simi- 

 lar construction is placed upon the top of it, and the 

 iron weights removed also to the top. In this man- 

 f V 



oal-ncld. 



