166 Mr. BuDDLE on Subsidences 



turn which it would otherwise do ; besides, the small portions of coal which have 

 been left occupy a certain part of the excavation when crushed down. 



These observations apply more particularly to the Newcastle system of working, 

 or " short work," where rectangular pillars are left, in the first instance, and after- 

 wards removed. 



In the working of those pillars, stooks, or blocks of coal of considerable strength, 

 are frequently left as props to protect the colhers from the exfoliations of the 

 roof. In this case, a subsidence of the strata above invariably takes place ; but its 

 extent in the first instance is governed by the degree of resistance which those 

 stooks of coal present to the downward pressure of the mass. When the sink- 

 ing of the strata is retarded or stopped in this manner, before the excavation 

 whence the coal has been extracted is entirely filled, the place is said to be " half 

 or three quarters, etc. crept." A large tract of a coal-mine not unfrequently 

 remains for several years in this state, without almost any perceptible change 

 taking place ; yet in course of time, from the exfoliation of the coal in those 

 " stooks," and the operation of the atmosphere, a further subsidence, called a 

 second creep, takes place, and generally continues until the excavation is com- 

 pletely closed. 



In the Yorkshire system of " long work," where the coal is all taken out in the 

 first instance, except small temporary pillars, in addition to which wooden props, 

 together with a sort of stone pillaring called " gobbing," are placed for the pro- 

 tection of the colliers, the subsidence of the strata immediately follows the removal 

 of the coal, and the excavation is completely filled, so that no second settlement 

 or set takes place. 



I have already observed, that the extent of settlement, as shown on the surface, 

 from working coal, depends upon so many contingencies, that it cannot be sub- 

 jected to any calculation. It rarely happens that the perpendicular depth of a 

 sinking of the surface above coal-workings can be accurately ascertained, as fixed 

 land-marks are seldom attended to in such cases. Railways, or reservoirs of water, 

 however, occasionally afford the means of ascertaining, with tolerable accuracy, 

 the extent 'to which the surface has subsided by the working out of the coal. 



From the formation of a pond of water upwards of three feet deep on a level 

 surface, from underneath which a six-feet seam of coal had been worked at the 

 depth of 100 fathoms, about one-fourth part of the seam having been left in 

 stooks, as already described, I ascertained that the subsidence of the surface at its 

 greatest depression had been rather more than three feet. In this case the sand- 

 stone strata predominated. 



The next case which I have had an opportunity of observing with tolerable 

 accuracy is a more interesting one, as it shows three distinct periods of subsidence, 



