92 An "Essay on Renh. (TeB. 



tion, and those which are bent towards the rent, are widest in the 

 middle, and meet at the highest and lowest points of the rent, so 

 the distances from the rent to which the strata are bent are greatest 

 opposite its centre, and decrease gradually to its highest and lowest 

 extremities. Again, as the sphere of the horizontal contraction has 

 extended as far as the strata are bent, and as the distance from the 

 rent to where the contraction in this direction commenced, is 

 greatest in the middle of the rent, and decreases upwards and 

 downwards from this jdace ; so the distance which the strata have 

 contracted horizontally is greatest at the middle, and decreases 

 gradually towards the highest and lowest extremities of the rent. 



From what I have seen of the smaller rents of this shape, in 

 general when the distanceytf is four yards, the distance rf c is two 

 feet, the distance v b DO yards, and the distance a b three yards. 

 From these data the contraction horizontally is one-sixth of the 

 distance in which that contraction took place. How much the 

 contraction is perpendicularly we have no direct means of ascer- 

 taining ; but that it is as much in this as in the horizontal direction 

 is extremely probable ; and on this supposition the distance which 

 the strata have contracted less on the under than on the upper side 

 of tliis rent is one-fifth of their whole contraction in this direction. 



The matter near this rent has contracted more horizontally in 

 one part than in another. Thus its horizontal contraction is greater 

 opposite the part h, fig. 3, Plate XXIX., than opposite the part c ; 

 and more opposite the part d than the part e ; and so on. The 

 inequality in the matter's contraction in this direction is from one- 

 fifth to one-sixth of its whole contraction. This inequality gives 

 the curvilinear bends to loth sides of a rent, which necessarily pro- 

 duce inequalities in its width. But that arrangement of these 

 inequalities represented by fig. 3 was formed by the inequalities of 

 the matter's contraction taken perpendicularly as well as horizon- 

 tally; because sunilar bends in the sides of this rent are never 

 wholly opposite each other, but always situated lower on the upper 

 side than on the under side ; and this arrangement is owing to the 

 circumstance before mentioned, that the parts of the strata on the 

 former side, or the side g m, have contracted more perpendicularly 

 or sunk lower than on the latter side, the side a f, of the rent. 

 When the hollow parts on the upper side of a rent have sunk 

 directly opposite similar hollows on the rent's under side, the varia- 

 tion in the width before described and represented by fig. 4, Plate 

 XXIX., is produced ; but corresponding hollows are not opposite 

 each other ; for the hollow a was originally opposite the hollow a 1, 

 &c. There are many other variations in the ratio of the strata's 

 bending, and in the regularity of the widths and positions of 

 bended-tabular rents, which depend upon certain modifications of 

 the principles already laid down. Some of these vaiiations I will 

 afterwards describe in separate essays. 



