46 Notices respecting New Books. 



by which we have another check on all preceding calculations. 

 The double areal velocity really taking place in the true el- 

 lipsis ^-', the time of revolution U, and the mean motion in 

 degree ju,' may be derived from these equations : 



rC — fC S6C Z J 



(41) ft' = - ,\, 360° = -4— 360°, 



^ ' '^ 2a'b'7r 2aOTr 



U = 



2a'i'7r 2a bi 



1<! -~ k ' 



in which the unit of time is that in which the observations have 

 been recorded, and the unit of space the second of an arc by 

 which the distances have been measured. 

 [To be continued.] 



VII. Notices respecting New Books. 

 A Geological Manual. By H. T. De la Beche, Esq. F.R.S. F. G.S. &c. 



IT is not long since Mr. De la Beche presented to the scientific 

 world a useful and instructive collection of " Sections and Views 

 illustrative of Geological Phaenomena*." To this publication he has 

 now added a more systematic and important volume, entitled, " A 

 Geological Manual," a work of first-rate importance in the science 

 to which it relates, and which must henceforth take its place in the 

 library of every student in geology. 



The author approaches his subject eminently qualified for the task 

 he has undertaken, having devoted many years of his life most assi- 

 duously to this engaging pursuit. His information on English geology 

 is founded on long-continued personal examination of the strata of 

 this country, and on a familiar acquaintance with all that has been 

 written respecting them : to this he adds an extensive knowledge of 

 European geology, collected during many tours on the Continent, and 

 from copious reading. He has further extended his observations to 

 tropical regions, by a visit to the West Indies, 



A vast and faithful accumulation of facts has resulted from the in- 

 dustrious use of such rare and precious opportunities. These have 

 been systematically arranged in the work before us, and upon them 

 the author has been cautious to found such conclusions only as he 

 thinks to be warranted by the premises before him. We consider the 

 additions he has thus made to our stock of geological knowledge to 

 be a very important step in our advancement towards a true theory 

 of the earth. 



The author commences his work with an account of the figure and 

 density of the earth, the superficial distribution of land and water, 

 the saltness and specific gravity of the sea, and the temperature of the 



• See Phil. Mag. and Annals, N.S. vol. ix. p. 131. 



earth. 



