1818.] Geological Society. 69 



A paper, by Sir W. Herschell, was also read, entitled astro- 

 nomical observations and experiments selected for the purpose 

 of ascertaining the relative distance of clusters of stars ; and of 

 investigating how far the power of our telescopes may be 

 expected to reach into space when directed to ambiguous celes- 

 tial objects. 



GEOLOGICAL SOCIETY. 



March 6. — A paper was read, entitled " Observations on the 

 Valleys and Watercourses of Shropshire, and of Parts of the 

 adjacent Counties," by Arthur Aikin, Esq. 



From the heights of parts of the line of the Ellesmere canal, 

 and from other data, Mr. A. computes the summit level of the 

 tract which separates the valley of the Dee from that of the 

 Severn, to be about 295 feet above the Dee at Chester ; and the 

 height of the Severn at Shrewsbury to be about 155 feet above 

 the Dee at Chester. 



The descent of the Severn from Llanidloes to the sea appears 

 to be at the rate of 1 1 feet per mile for the first 20 miles, not 

 navigable ; then three feet eight inches per mile for 26 miles ; 

 one foot eight inches per mile for 21 miles, and from Worcester 

 to Gloucester, about 30 miles, only four inches per mile. 



From a variety of observations on the course of the Severn, 

 Mr. A. concludes that the navigation of a river is very precarious, 

 and liable to long and frequent interruptions, even in a rainy 

 climate, when the descent of the stream exceeds three feet per 

 mile, and that the highest floods run off in a few hours, even 

 when the descent amounts to two feet six inches in the same 

 space. 



The descent of the Dee from Llandysilio to Pont y cysyllte, a 

 distance of six miles, is at the rate of 22 feet per mile ; and 

 thence to Chester, amounts to about five feet one inch per mile. 

 The heights of water-sheds, or sources of rivers, being import- 

 ant points in physical geography, Mr. Aikin recommends the 

 subject to the notice of such members of the Society as may be 

 enabled to supply information concerning it. 



April 3. — A paper was read, from Dr. Brewster, on the form 

 of the integrant molecule of carbonate of lime. 



Dr. B. has discovered that the striae passing through the long 

 diagonals of two opposite planes of the primitive rhomboid of 

 carbonate of lime are occasioned by their traversing veins com- 

 posed of rhomboids of different thicknesses, having their faces 

 placed transversely to those of the rhomboid which they traverse, 

 and adhering firmly to the two surfaces between which they are 

 interposed. Dr. B. rests the proof of this fact on the action 

 which the surface of the crystal exerts on a ray of light. He 

 concludes that the integrant molecule is not a trihedral prism, 

 as Count Bournon supposed, since the transverse cleavage of 

 the primitive rhomboid exists only in those specimens which are 

 crossed by intersecting veins. 



