1815.] An Essay on "Rents. 89 



Many instances might be brought forward to show the cause of the 

 unequal contraction of every stratum ; but the one adduced is suffi- 

 cient to establish the principle. Having made these premises on 

 the nature of the unequal distribution of the elementary matter of 

 strata, I will now show the mechanical parr of the process by which 

 it produced the peculiar arrangement of the strata near the bended- 

 tabular rent ; and, in consequence of it, the formation of this rent. 

 Suppose the state of solidity of the matter such that the forma- 

 tion of rents had commenced, and that the rent, D, fig. 2, Plate 

 XXX., was produced in a place where the matter's attraction of 

 cohesion was least able to resist the separating force. Let the dotted 

 lines, r s, V w, z a\, be the upper sides of the strata ABC, 

 when the formation of the rent, D, commenced ; and the lines 

 pfe b c q, t m k g h u, and x \ 2 3 4;/, those of these of the 

 strata when they bad acquired their present state of solidity. The 

 formation of the rent first commenced in the stratum A : and after 

 the surfaces of separation were formed, the part b c contracted 

 perpendicularly less, and the part f e a very little more, than did 

 the stratum A, in general. In consequence, then, of the con- 

 traction, horizontally and perpendicularly, of this stratum, the 

 rent a e was produced ; whose under side, b d, is at right angles 

 to the part b c, and whose upper side, a e, is at right angles to the 

 part ye; and as the end, e, of the part ye is not so far below the 

 line f c as the end, /', of the part ^ c is above it, the angular 

 direction of the sides, b a and e d, of the rent, D, meet at the 

 point, d, from a width, e d, at their upper extremities that is 

 nearly equal to the horizontal contraction of the part f c of the 

 stratum A. The arrangement of elementary matter in the stratum 

 B being similar to its arrangement in the stratum A, a rent would 

 take place in the former immediately above that in the latter 

 stratum. Like the stratum A, the stratum B would contract less 

 in one part, g It, than in another, m k ; but although its thickness 

 be the same as that of the stratum B, the distance g i is twice the 

 distance b d; because, besides that it has contracted as much less 

 in that part than in other parts, like the stratum A, the progress 

 towards the present state of solidity of the stratum A was greater in 

 the same period of time than that of the stratum B ; consequently 

 the part c b g k of this stratum was raised as much above tlie line 

 d c as the line ^ c is above the liney r, or as much as it contracted 

 less in that part than in general. In such a manner were the parts 

 t,r,l), b, of the strata H, (i, F, A, fig. 1, Plate XXIX., raised 

 ahove the other parts of the same strata. Again, the distance 

 which the point Ji is below the line tt? // ?/, fig. 2, Plato XXX., is 

 twice that whicli the pciint e is below the YiwcJ C; i)ecausc the part 

 m k e f of the stnilum B has contracted as much as the same part 

 iA the stratum A, and is as much let down below the other parts of 

 the stratum M, by the unequal contraction of the stratum A, as the 

 point i; is helow the line j^'r;. In (his manner were the parts //, .«, 

 q, a, of the strata H, G, F, A, fig. 1, Plate XXIX., allowed to 



