Notices respecting New Books, 289 



Weight of the needle complete, with the forty magnets, the small 

 segments cut from leaden sphere, and the mirror, 

 3-4 grammes, 

 v = 299-5 million metres per second. 



June 25th. 

 A = 1323-14 square centimetres, 

 t = 0-7728 centimetre, 



P= 42-2 seconds, 

 r 1= = 3-0045 ohms, 



^ + ^=10040 

 E= 12000 



8 _ 1 



s+g 1000' 



d t == 263 scale-divisions, 



d. 2 — 223-9 „ „ mean of 18 discharges, 

 j\= 0-081865. 



Weight of the needle complete, with the forty magnets, the small 

 segments cut from the leaden sphere, and the mirror, 

 3-4 grammes, 

 v — 297*2 million metres per second. 

 Mean of the three values of v (i. e. the final result from the 

 ninety-eight discharges of the air-condenser) is 

 298 million metres per second, 

 or exactly the velocity found byM. Foucault/or light. 



The probable error of our answer (298) is about 1 per cent. Now 

 the difference between M. Foucault's velocity for light (298 million 

 metres per second) and M. Cornu's (300 million metres per second) 

 is less than 1 per cent. We may therefore conclude that these two 

 velocities for light, as well as the value we have obtained by the 

 method which theoretically ought to give the most accurate deter- 

 mination for the ratio of the electromagnetic to the electrostatic unit 

 of quantity, are all equal within the limits of our experiments. 



XLV. Notices respecting New Books, 



The Study of Rocks, an Elementary Text-book on Petrology. By 



Frank Eutley, F.G.S. London: Longmans and Co. 1879. 



HPHE study of rocks has of late years assumed considerable 

 -*- interest and importance. Their classification, based on their 

 different origins, modes of occurrence, or mineral character, has 

 been now further supplemented by an examination of the micro- 

 scopic structure. 



In this latter field of inquiry the suggestive paper by Mr. Sorby* 

 in 1858 led the way, which was afterwards followed by the elaborate 

 researches of Zirkel and those of Rosenbusch, Vogelsang, von 

 Lasaulx, Boricky, and other continental petrologists, while in our 

 own country the labours of Allport, Bonney, D. Forbes, J. A. 



* " On the Microscopical Structure of Crystals," Quart. Journ. G. S. xiv. 

 p. 453. 

 Phil. Mag. S. 5. Vol. 7. No. 43. April 1879. Z 



