1 1 8 Annals of the Philosophical Club 



heights. When asked what allowances were made for 

 local causes, such as the presence of clouds, he replied that 

 exact comparisons had not yet been made, but that observa- 

 tions, taken at the Convent nine times daily by competent 

 scientific men, promised to be of considerable value. 



Mr. Grove said that he had found by experiment the ad- 

 vantage of resinous compounds as a substitute for flint glass to 

 secure achromatism in telescope lenses. The best ingredient 

 was a cement formed by nearly colourless resin and castor 

 oil, the refractive and dispersive powers of which could be 

 altered within certain limits, and which was sufficiently 

 pliant and tenacious to yield to the expansion of the glass 

 by temperature. Among other promising experiments, 

 he had very satisfactorily corrected a bad lens by using this 

 composition. That was done some months ago, but he 

 mentioned the matter, because he had read in a Catalogue 

 of the Exhibition 1 that a telescope there had a solid material, 

 the composition of which was kept secret, instead of flint 

 glass. M. Plantamour said that oxide of zinc had been 

 substituted for oxide of lead in flint glass, and that glass 

 so made was less liable to corrosion by exposure to the 

 atmosphere. 



Mr. Horner referred to investigations in the Nile Valley, 

 undertaken to ascertain the rate at which alluvium had 

 accumulated, by ascertaining its thickness around monu- 

 ments, the date of which was known, and thus to connect 

 historical and geological time. It was intended to begin 

 at the Obelisk of HeHopolis, the age of which was known. 2 



At the 4ist meeting (Oct. 3Oth) Captain Skogman, of the 

 Swedish Navy, who was a guest, announced that the measure- 

 ment of an arc of the meridian was about to be repeated 

 in his country, and observations were then being taken to 

 ascertain whether the Baltic and the Polar seas differed 

 in level. 



To the 42nd meeting (Nov. 27th) Mr. Bell gave an account 



1 The well-known one opened May ist, 1851. 



2 See Lyell, Antiquity oj Man, ch. iii. (especially pages 40, 41, ed. 4), 

 and Principles of Geology, vol. i. pages 430-434, ed. n. 



