M I N E. 



343 



Minei of is regularly wedged with one set of wedges all around ; 

 Cnl. tnen w ith a second and third set in the same regular 



^""T~~" manner, the i>'>ject being to keep the crib in a posi- 

 |,Jl5.* tion truly circular, the wedges used in this operation 

 are made of dry clean fir very thin at the head in 

 comparison with their length, openings are made for 

 entering the points of these wedges by a steel wedging 

 chissel ; and the wedging is continued regularly around, 

 until the steel wedging chissel cannot make an opening 

 in the wood. By this operation no water can pass 

 downwards by the back of the crib. The next opera- 

 tion is to fix a spiking crib, /; to the rock, about 10 or 

 12 feet from the lower crib, according to the length of 

 the planks to be used for the tubs. This spikin _ 

 is about five inches square, and is only used for spiking 

 the deals of the tub t.i it. They must, however, be set 

 true to the iweep of the pit, a* upon them the true cir- 

 cular figure of it depends*. This spiking crib being 

 placed, the tubbing deal, A, are to lie fixed. These are 

 made of deals three inches thick, and about six inches 

 broad, planed on all aides, and the joint* wrought to 

 the true bevel of the circle of the pit Sheeting deals 

 are laid under the deal end*, and the tub deals are firm- 

 ly set together, one by one, in a perpendicular direc- 

 tion, thew upper ends reaching to the middle of the 

 spiking crib /. Formerly these planks were fixed to 

 the spiking crib* with iron nail* or spikes, but a* these 

 were soon destroyed by corrosion, particularly when 

 there was any salt in the water, treenails are now used, 

 and found to be much better. When the planks are 

 set around, and within less than three breadth* of a 

 plank from being closed, a plank i* placed on each 

 side of the opening, having a square joint next each 

 other in place of beinp bevelled, which leave* a rectan- 

 gular or parallel space for inserting the closer or closing 

 plank. This plank is> prepared so as just to enter by 

 force, it is then rubbed on the edges with soft soap, 

 and is, by main force, and heavy mallets, driven flush 

 with the other plank. The sheeting deals under the 

 ends of the plank, and in the joints of the wedging crib 

 are then wetlged tight, in the manner before mentioned. 

 The main crib* gg, arc then to be placed as counter- 

 fort*, for the security and strengthening of the tubbing. 

 These crib* sire made in the manner before mention- 

 ed ; they are about eight or nine inches in the bed, 

 seven inches thick, and are placed at distances from 

 each other, proportioned to the column of water to 

 be resisted. At greet depths they are laid close toge- 

 ther, and the distance from each other increases ae the 

 pressure decreases, care being taken to have always a 

 deckled excess of strength, and made to bear equal- 

 ly against the tub deals, by means of wedging at the 

 joints. The first main crib is laid close to the foot of 

 the tub deals, and resting on the wedging crib. Dur- 

 ing this operation, the growth of water at the back of 

 the tubbing is allowed to flow freely through a plug 

 hole at the foot of the tubbing, until the whole opera- 

 tion is completed. The upper ends of the first set of 

 tab plank* are then cut square, and level all around. 

 The second spiking crib /, is then fixed, and another 

 set of tubbing desJi put around as the former, having 

 sheeting deal inserted betwixt the ends of the two sets 

 at/. When this is wedged, the cribs h, h, are placed 

 in the manner before described. . According to this 

 plan, tubbing may be carried upwards to any height, 

 until the water ruing at the back of it would discharge 

 itself into the natural reservoir above the rock bead, as 

 i* frequently done ; but if a tubbing of a few fathoms 

 is to be formed in a recess at a considerable depth from 



Mix* of 

 Coal. 



the rock head, then a wedging crib must be placed at 



the top of the recess, and firmly back-wedged, as at the 



lower wedging crib d. When this kind of tubbing is ,^! < T~~ 



completed, the plug hole is filled up, and the water coaT-fi'ela. * 



quickly rises and produces its full pressure. Although, 



from the nature of the operation described, it might be 



supposed that nothing could shrink even in the least 



degree, yet, as the pressure accumulates, the wood 



and the joints are heard very distinctly creaking by the 



excessive pressure. From this it is evident that no 



form but th.it of a circle could resist such a weight. 



After the full pressure is obtained, it frequently happens 



that veins of water, not thicker than a fine hair, will be 



found springing from the sides 'of the tubbing ; these 



generally take up in a few days by the swelling of the 



wood. When all is secured, sheathing deals, inch and 



quarter thick, are nailed to the cribs all around the pit, 



not only to preserve the cribs from injury, but to make 



a smooth wall for the ascending and descending baskets 



with coals. These sheathing deals are put on in regular 



perpendicular courses, and the but-ends meet flush on 



one of the cribs. 



The solid cribbing, executed with oak cribs only, and Solid crib- 

 no tubbing deals, ia a more complete piece of work bing. 

 than the former. If the pressure is great, they are 

 formed three to four feet long, ten inches in the bed, 

 and seven or eight inches deep, of the best oak, as be- 

 fore described. The first crib is laid down, and back 

 wedged, in the same manner as the wedging crib in the 

 plank tubbing, with this difference, that it has been 

 found that when the wedging was made in lines paral- 

 lel to the sweep of the cribs, the immense pressure had 

 a tendency to press forward the lower part of the crib ; 

 and it ha* been found from experience, that, if the 

 wedging is done in a diagonal manner, it is much su- 

 perior, and auite effectual. After the back wedging is 

 completed, the next tier of cribs is laid down in precise 

 position, having sheeting deal along the bed, and at 

 each end joint, the perpendicular joints are overlapped 

 in each set by the position of the next segments alter- 

 nately, so that the face of the cribs, with the jointings 

 of sheeting deals, has the appearance of ashler work. 

 A wedging crib is fixed in the upper tier, if next the 

 top of the recess, and the back wedging completed, as 

 before directed. Each crib is set true to the radius of 

 the pit, and the back of it filled up with pieces of wood 

 betwixt and the rock. When the whole space proposed 

 to be cribbed is filled up, then the wedging of the joints 

 commences while the water flows by a plug hole at the 

 bottom. All the horizontal and perpendicular joints 

 are gone over completely with one set of wedges ; and, 

 in the same way, a second and a third set of wedging is 

 done, till wedge* can be no longer driven. The face of 

 the wedgings is then made smooth, the plug hole filled 

 up, and the full pressure sustained. In this kind of 

 work, innumerable fine veins or filaments of water issue 

 from the wedging*, the water being forced through 

 the pores of the wood. If these do not take up, addition- 

 al wedging must be attempted, till not a panicle of wa- 

 ter can be seen issuing. In this piece of work, much 

 ingenuity is displayed, and it reflects much credit upon 

 the inventor. 



The third mod* of tubbing is by means of iron cy- i ron t j. 

 linders cast into segment*. These are different from linden. 

 the drop or sinking cylinders, because they are placed 

 piece after piece in the circular recess of the pit cut out 

 for them. The flange for the wedging joint is some- 

 times toward the centre of the pit, but the improved 

 method is to turn the flange inwards. In the latest 



