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Mk HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



23. Let AB represent the fissure. All, BV, the physical lines 

 perpendicular to it, and passing close to its extremities, and for the 

 greater distinctness, let us suppose the boundary of the lamina along 

 UV to be parallel to the fissure. Let EF be a physical line originally 

 parallel to the straight line AB. After the formation of the fissure, 



it will evidently assume a curved form resembling that of APB; but 

 its curvature will be less than that of the latter line, since the curva- 

 ture of all such lines must obviously be smaller the nearer they are 

 situated to the fixed straight line UV, along which it becomes evanescent. 

 If, however, the length of the fissure be considerable, the curvature of 

 APB will be very small, and therefore the variation of curvature in 

 successive physical lines such as EF, will be extremely slow, AU 

 being very large*. 



Also, let PQS be a physical line, parallel before the opening of the 

 fissure, to AU. If the form of every such line as EQF were exactly 

 the same, this line would still be accurately straight and parallel to 

 AU, and consequently in the case we are supposing it will be ap- 

 proximately so. The tension of all such lines will evidently be much 

 affected by the opening of the fissure. Since there is no force acting 

 at P, the tension of SP in the direction of its length, will, at that 



* If the boundary of the lamina be not parallel to the fissure, UF may be con- 

 ceived to be a physical line in the lamina, vert/ distant from and parallel to the line AB, 

 previously to the formation of the fissure, since the position or rectilinearity of such a 

 line will not be sensibly affected by the opening of the fissure, as appears from the text. 



