140 Proceedings of Philosophical Societies. [Aug. 



with respect to the acceleration of the pendulum between 

 London and Unst, agreed with his own to within 0-6". 



July 1. — A paper was read, on the causes which influence the 

 direction of the magnetic needle, by Capt. J. Burney, R.N. The 

 author, after relating a variety of interesting experiments, 

 appeared to conclude that the compass is governed partly by 

 polarity, which he considers as created by motion, and the pri- 

 mary cause of the needle's pointing north and south, and partly 

 by attraction which is inherent in matter ; the former of which is 

 constant, the latter variable. The author attempted to explain 

 on these principles why the needle is most liable to be disturbed 

 in high latitudes by attraction, the obliquity of the plane of 

 the earth's rotatory motion to the horizon being here greater, and 

 thus the polarity of the needle from this cause being diminished. 

 At this meeting also a paper was read, by Arthur Jacob, M.D. 

 of Dublin, giving an account of a new membrane discovered in 

 the eye. The author described a delicate transparent membrane 

 covering the external surface of the retina, and united to it by 

 cellular substance. The paper was concluded by pointing out 

 the best method of detecting and examining it. 



The titles of the two following papers were also read, which, 

 from the nature of the subjects, did not admit of being read in 

 detail. 



" On the Theory of Capillary Attraction," by J. Ivory, Esq. 

 " On a new Method of solving Numerical Equations of all 

 Orders by continuous Approximation," by W. G. Horner, Esq. 

 The Society adjourned till November. 



GEOLOGICAL SOCIETY. 



March 19. — A paper was read from George Cumberland, Esq. 

 on some new varieties of encrini and pentacrini. 



A communication was received from the Rev. Richard Hen- 

 nah " On the Calcareous Rocks of Plymouth, containing Re- 

 marks connected with, and illustrative of, their natural History." 



April 2. — The reading of Mr. Hennah's paper, on the Plymouth 

 limestone, was concluded. 



This bed rests generally on clay slate, and rises about 100 

 feet above high water mark ; it runs nearly east and west for 

 several miles, and dips towards the south or south-west; its 

 breadth is from a quarter to half a mile. 



Westward it does not appear much below Mount Edgecumbe ; 

 eastward the author supposes it to approach Dartmouth. 



The character of the bed near Plymouth is various in colour, 

 considerably compact, and containing many organic remains, 

 chiefly, madripores, tubipores, millepores, trochites, pentacri- 

 nites, vertebral columns, corallites, varieties of bivalve and 

 univalve shells, but seldom found together on the same part of 

 the bed. Varieties of calcareous stalactites and of crystallized 

 carbonate of lime have also been found. 

 5 



