1815.] ^n Essay on Rerits. 91 



])resent the angular direction of its upper half present a similarly 

 disposed curve as its lower half, but it has a reverse direction ; and 

 the width of the upper half decreases from its maxinmra at the 

 middle to a point at the highest extremity of the rent. 



As the part of any one of the strata on the under side of this 

 rent below the stratum A has raised that which lies upon it as much 

 as it was itself raised by that on which it lies, with the additional 

 distance of what it contracted less than that produced by the general 

 ratio of contraction for a distance equal to its tluckness ; and as its 

 correspondent part on the upper side of the rent has let the stratum 

 immediately above it decend as much as its own descent below the 

 general line of the stratum, with the additional distance of what it 

 contracted more than the strata in general ; it follows that the 

 distances between the surfaces of separation of these strata are the 

 respective measures of the matter's unequal contraction between 

 them and the lowest extremity of the rent. Thus the distance, a b 

 between these surfaces of the stratum A is tiie measure of the 

 unequal contraction of the matter between that stratum and the 

 lowest extremity, v, of the rent. The distances p q, r v, t u, be- 

 tween separated surfaces of the strata F, G, H, are the measures 

 of the matter's unequal contraction between these strata and the 

 rent's lowest extremity respectively ; and as much as the distance 

 between the surfaces of separation of any one of these strata and 

 the point v is less than the distance between the separated surfaces 

 of the stratum A, so much is the distance of this stratum from the 

 point V less than that of the stratum A. Again, as tiie part of any 

 one of the strata which are situated above the stratum A on the 

 under side of this rent has raised the stratum lying upon it as uiiich 

 as it was itself raised by that which lies below it, with the deduction 

 of the distance of what its ratio of contraction is more than the 

 general ratio ; and as its correspondent part on the upper side of the 

 rent has permitted the stratum immediately above it to descend a 

 distance equal to that which itself descended below the general line 

 of the stratum, with the deduction of what its ratio of contraction 

 is less than the given ratio; it follows that the distance between the 

 surfaces of separation of any one of these strata is the measure of 

 the unequal contraction between that stratum and the bottom of 

 the rent ; and the difference of the distance between its surfaces of 

 •eparation and that between the separated surfaces of the stratum A 

 is the measure of the decrease in the amoimt of the unequal con- 

 traction lietween that stratinn and the stratum A : the first of which 

 ditierences increases, and the second decreases as the distance of 

 each from the stratum A increases, but both cease with the rent. 

 The power, thercibrc, of the unequal perpendicular eontraclion of 

 the matter to displiiee the strata comnieiices at the rent's lowest 

 extremity, accumnlutes in eir<'Ct from that place (o the middle hut 

 Irom tliencc decreases upwards, and teases with the rent. 

 ^ As the lines, n fv and vzit, which represent the lines of dis- 

 tinction between the parts of the strata that have the leguhir posi- 



