49 



pitate of phosphate of lime ; and in the remainin}; fluid, oxalate o am- 

 monia throws down oxahite of lime. The matter left undissolved by 

 the muriatic acid is inflammable, leaving a small siliceous residue, 

 and appears to be bituminous matter derived from the matrix. There 

 is no magnesia, sulphur, nor fluorine. 



The analysis of two coprolites, measuring from two inches to two 

 inches and a half in length, and containing each a few fish scales, gave 

 the following numerical results. 



Phosphate of lime with a little oxide of iron. 85.08 83.31 



Carbonate of lime, 10.79 15-11 



Silica, 0.34 0.29 



Bituminous matter, .... 3.95 1.47 



100.2 100.00 



The proportion of phosphate of lime, appears thus to be pretty uni- 

 formly 5-6ths of the whole. The variation in the proportion of car- 

 bonate of lime may probably be influenced by the matrix, from which 

 also the bituminous matter is derived. The limestone, when dissolv- 

 ed in muriatic acid, leaves a dark bituminous matter, in the propor- 

 tion of 2.5 per cent. 



t 



2. Notice relative to the Polyzonal Lenses belonging to 

 the Commissioners of the Northern Light-houses. By 

 Alan Stevenson, Esq. 



These lenses, which were exhibited to the Society by Mr Steven- 

 son, are three in number. One, a plano-convex lens, two feet six 

 inches square, was made by M. Soleil at Paris, under the superinten- 

 dence of the late ]M. Fresnel. Another is a double-convex circular 

 lens of flint-glass, three feet in diameter, which was constructed by the 

 Messrs Gilbert of London, at the suggestion of Sir David Brewster. 

 The third is a circular plano-convex lens, two feet six inches in dia- 

 meter, cast in one piece as originally proposed by BufFon. This lens 

 has been executed for the first time by the Messrs Cookson, plate- 

 glass-makers, Newcastle. 



The author, with the assistance of Mr John Adie jun., made a 

 numerous set of observations, to determine the relative value of these 

 lenses, and for this purpose ascertained,yjVif, the mean focal distance 

 of the central lens, and the several concentric rings of each, compa- 

 ring the results with the focal distance of its aggregate surface; and 



