164 On the Flexibility of 
ces, and forced out sideways till the hollows 
are filled with coal, and the matter of the 
pavement; lastly, it only remains for the pres- 
sure to squeeze the loose matter as firmly as 
it can. Now, by the time that the coal is 
separated from the weak parts of the pillars, 
the sinking of the roof-strata has become evi- 
dent ; and it continues till the stratum next 
above the coal, is only from one half to one 
third of the original height of the coal from 
the pavement. During this process, the first 
stratum above the coal, or if athick one, the 
lower part of it, bends downwards till it is 
rent from that above it; but the coal pillars 
not being able to support the disengaged lay- 
ers, it sinks, and leaves the second layer or 
stratum at liberty to bend; this layer now 
bends till it leaves the third, and adds to the 
weight that the coal pillars have to bear. In 
this way the separation of the strata goes on, 
while it can be heard in the hollows of the 
coal works: when for instance, one layer be- 
gins to separate, a small part only is detached 
at once, with a noise that coaliers call a thud. 
The thuds shift their situations at every repe- 
tition of sound: sometimes they move alter- 
nately, from dip to rise; they often spread in 
every direction from the weakest part; they 
