166 On the Flexibility of 
soon yield alittle, room isagain made for the 
then firm strata to bend, and to separate ; 
therefore, the sinking recommences, and con- 
tinues, with such like interruptions, till the 
pillars are squeezed out and fill the hollows. 
The layers after being once separated, can- 
not be forced again into their original space ; 
hence the sinking has a limit, and if the dis- 
tance between the coal and the surface be 
more than that to which the sinking can ex- 
tend upwards, the surface strata are not acted 
on. But if the sinking do reach the surface, 
sinall hollows appear which slowly expand, 
walls separate into parts and fall down, and 
streams of water enter into small upright 
seats instead of continuing in their usual 
course. But the sinking acts upon houses in 
the most extraordinary manner: the windows 
break without any visible cause ; the window 
stones crack; afterwards the house sides are 
rent ; and, if the creep be strong at the sur- 
face, a part of, if not all, the house falls to 
the ground. 
When the coal mine has been excavated 
very fast, and most of the coal taken away, 
a rapid sinking of the roof strata is sure to fol- 
low soon after. ‘This sinking is called a set 
or seat by the coaliers, and differs only from 
