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214 An Essay on Rents. + {Sepr, 
is the general mode, and one that is subject to some modifications, 
there is another mode by which this difference of level is produced. 
Miners often find the strata, any distance from a foot to more 
. than 40 fathoms asunder, in point of altitude, on opposite sides of 
bended-tabular rents, in places where these strata are not bent; 
out straight, as the stratum abcd, Plate XX XVIII, fig. 1, and the 
stratum c, along with the strata B A, fig. 2, are represented. In 
the first mode, the distances which the strata are asunder, on oppo- 
site sides of these rents, are obtained by the bending of the strata, 
and always continue, and sometimes increase, during many tem- © 
porary suspensions of this bending ; but in the second mode, it takes 
place without any bending of the strata. 
I will give one example to illustrate each mode. Let fig. 1 be 
representative of the first mode. ‘The stratum ad is horizontal, 
notwithstanding the part a is the distance l c below the part c d. 
Above this stratum, at the rent A, the strata are straight for a certain 
distance, and then they are bent; and below this stratum they are 
also bent, at first slightly, but with a gradually increasing ratio, that 
reaches its maximum at the stratum k x; whose two parts & / and 
mn are the distance m J asunder, which is equal to the distance f g, 
and to the distance 4c, and which is acquired by the bending of the 
part mm above the line /. ‘The strata close to the side p gc at 
this part of the rent are thicker than close to the side / fl: they are 
also thicker at m p than at qz, and that additional thickness 
throughout the whole of the rent, below the stratum km, gave rise 
to the bending of the part m 7 of that stratum, in the manner which 
has been shown in my first communication on rents ; but the strata 
above the stratum mm 2 where close to the side m c of the rent, 
though thicker than the opposite strata on the side / L, are not so 
thick as they are at the line  d; in consequence, the bending, as 
seen at m 7, gradually decreases upwards till it ceases. Let us take, 
by way of illustration, the effect on the stratum c h, of this altera- 
tion in the thickness of the strata : as much 4s the strata which are 
situated between the strata k 2 and ch are thicker at the line » 4 
than at ke, so much is the distance z / greater than the distance 
k e, (say by the distance  0,) and so much is the stratum g / bent 
less than the stratum m 7, say by the distance ir. The bending of 
_ the strata above the stratum ch also diminishes upwards, from the 
same cause, till it ceases at the stratum a d, which is straight. At 
first sight the position of the stratum a d, considering how much 
one part is higher than the other, appears to be irreconcileable wit 
that arrangement which 1 have considered the general one; but 
when its connexion with that of the strata below is traced as we 
have now done, its difference from that which is the common one 
is easily accounted for. In fact, the arrangement of the elementary 
matters in this part is such, that the strata have contracted less, 
instead of more, at the line md than close to the side mc 
rent ; and by doing so have gradually given the straight, instead of 
‘ . 
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