Mr. M. Dunn- on the Sinking of Preston Grange Engine Pit. 233 



insert a casting, to occupy the space of four segments, viz. : — 8 feet long 

 by 4 feet high, containing an egg-shaped bulb, 5 feet by 8 feet, projecting 

 outwards, wherein a man might station himself to change the clack,* 

 the flanches at each end of this being contrived so as to break 

 the joints all around the pit. Considerable apprehension was entertained 

 of this tub standing good against the extreme pressure which was in 

 this case laid upon the point of junction of the two sweeps. In the 

 first place the common pressure upon every square inch of the bulb 

 was 861b. per inch, but the aggregate pressure upon the bulb 5 

 feet by 3, 65 tons devolved upon that part where the two sweeps met. 

 No sooner was all finished and the water had acquired nearly its full 

 pressure, then the mouth of this bulb began to shew symptoms of weak- 

 ness by a crack of 12 inches in length. The pressure was immediately 

 eased, and no time lost in preparing four inside segments of cast iron, 

 8 inches broad and 2 inches deep, fitted accurately to the sweep of the 

 orifice of the bulb, which, being keyed closely up, completely answered 

 the purpose of strengthening every thing equal to the required pressure. 

 The second column of pumps now became the hanging set, and the 

 sinking proceeded to the depth of 44 fathoms, where 4>i fathoms of tub 

 were again required to get rid of water to the extent of 350 gallons per 

 minute. The segments of this tub were increased in thickness to f and f, 

 with deeper flanches. The top and bottom wedging cribs were proceeded 

 with in the same manner as before described, but the job was rendered 

 difficult of completion owing to a considerable mixture of carbonic acid 

 gas being contained in the water, which affected the eye-sight of the 

 sinkers, and so operated against the tightening of the tub that at one 

 time it was feared air pipes would have been necessary from hence 

 to the surface to complete the safety of the pit. 



As all these waters have their origin on the surface, and as they rise 

 considerably above the level of the shaft, I calculate the pressure upon 

 this last-mentioned tub at 310 feet perpendicular, or 1331b. upon every 

 square inch of its surface ; and as the outside surface of that tub con- 

 tains 768 square feet, it is therefore sustaining a pressure of 6313 tons. 



* See Plate IV. Fig. 2. 



