S4 ^ft Essay on Rents. [Fe»» 



and other parts vary much from that general increase which begins 

 at their lines of teimination and ends at their centres. 



Both directions of this rent are crookedly and curvilinearly bent. 

 I will describe two of the principal bends. Let the dotted lines, 

 a, 0, fig. 2, Plate XXIX., represent the angular direction of a part 

 of this rent, which being a small part of the whole, reduced to so 

 small a magnitude, when compared with its real size, the curve 

 peculiar to this direction cannot be observed. Suppose the rent's 

 angular direction in the first part make an angle, c d a, of about 

 ten degrees with its general direction, a b ; h may then be parallel 

 to this direction a greater distance, d c ; then incline to, and cross 

 it, making with it as small an angle as before, during a greater or 

 less distance, of; it then inclines to and re-crosses it, making only 

 a small angle with it, for a considerable distance, fl;. These bends 

 are common to the horizontal, as well as the angular, direction of 

 this rent, and they may be thus accounted for : The parts which 

 difi'er in appearance, consistence, &c. from the characteristic parts 

 of the strata, originally laid in these crooked directions; and as 

 these parts caused the rents in a manner to be afterwards described, 

 the directions of these rents must necessarily follow those of such 

 parts. Both directions of these rents are curvilinearly bended. 



generally as fig. 3, Plate XXIX., represents. 

 The variations ii 



in the width are mostly similar to those described 

 as follows : The width of this rent will often decrease opposite a 

 few strata till the sides are nearly in contact ; it will then increase 

 opposite a few more strata, till it be as great or greater than before ; it 

 will then decrease and increase alternately, with different ratios, 

 during which its sides will occasionally meet, and remain in contact 

 for variable distances. The widths of both directions of a rent are 

 ^ subject to these variations ; and either of them will be well repre- 

 ' seated by fig. 4, Plate XXIX., which in this instance is a part of 

 the angular direction. 



3. The Dhncnsmis of Bended-Tabular Rents. — Bended-tabular 

 rents exist of all dimensions, from a few feet to several hundred 

 yards in both directions ; but their widths are generally from two to 

 four feet, and seldom exceed four yards, although there are a few 

 instances in Cornwall of their swelling out to ten yards. The fol- 

 lowing may be considered large rents : Two or three rents have been 

 traced in, and beyond, the Whitehaven coal-mines about 3000 

 yards in tlieir horizontal directions ; and one called Saltom North 

 Dyke is known to be more than 550 yards in its angular direction ; 

 but how much more cannot be now ascertained : and Professor 

 Jameson mentions two near Freybcrg, in Germany ; one of which 

 is more than four miles, and the other more than ten miles, in this 

 direction. Now the extent of the largest mine, whether one of 

 those which are used for the abstraction of ores out of rents, or 

 useful minerals from tlie strata themselves, is far from being equal 

 to the larger of these rents j of course, the observations of miners 



