54 Analyses of Booh. [July, 



The observations are made with two needles : one of them is a 

 cylinder, terminating at each end in a cone; the other is of the 

 usual shape, a slender parallelopipedon, and weighs 48 grains ; 

 the weight of the former is 65 grains. These two needles, in 

 general, give the same variation, seldom differing from each 

 other more than one minute, and the mean of both is considered 

 as the true variation. From the observations hitherto made, the 

 least variation is between the hours of eight and nine in the 

 morning; it then increases till a few minutes before two in the 

 evening, and afterwards decreases; but at what hour the varia- 

 tion arrives at the same point it stood at in the morning is 

 uncertain ; but, from several observations, it is after eleven at 

 night. It occasionally happens that the needle has a lateral 

 vibratory motion ; and, as far as I am able to judge, it happens 

 on those days when the air has a peculiar softness ; the degree of 

 this motion does not exceed one minute and a half. 



It is singular that on the 6th inst. the variation in the evening 

 observation was greater than that at noon. 



Rain between the 1st of May and the 1st of June, 2 - 118 inches. 



Article XI. 

 Analyses of Books. 



I. Memoirs of the Literary and Philosophical Society of 

 Manchester. Second series. Vol. II. 1S13, pp. 484. 



{Continued from Vol. I. p. 465.) 



9. Cursory Remarks on the Mineral Substance called in Der- 

 byshire Rotten Stone.. By William Martin, F.L.S. &c] This 

 mineral is considered as a variety of tripoli, and is used for the 

 same purposes. It occurs on Bakewell Moor, in Derbyshire, and 

 in some other places ; and the pits are only opened every third 

 or fourth year, according to the demand. It lies over a kind of 

 limestone, containing animal remains, which Mr. Martin calls 

 black marble ; and he thinks it has been formed by the decom- 

 position of this marble, and the abstraction of the calcareous 

 particles. Rotten stone consists chiefly of alumina, with a little 

 silica, and bitumen. Carbonate of lime and oxide of iron are 

 occasional, but not constant ingredients. The following are two 

 analyses of it by Mr. Martin : — 



