Reviews — Prof. Poynting — Mean Density of the Earth. 175 



republication of Prof. Hall's descriptions and diagrams of the 

 Lower Carboniferous Crinoidea with additional comments thereon 

 and three tine plates of the type specimens, some of which are now- 

 illustrated for the first time. The text, intercalated by numerous 

 cuts, treats of the Platijcrinidce, Actinocrinidce, Cijatliocrinidcs, and 

 a blastoid {Orphocrinus Whitei) from the Burlington Limestone of 

 Iowa, Missouri, and Illinois and the Keokuk Limestone of Iowa. 

 Mr. Whitfield follows Messrs. Wachsmuth and Springer's " Revision 

 of the Falceocrinidce " in generic nomenclature, but takes the oppor- 

 tunity to rectify some misapprehensions in regard to certain types 

 chiefly arising from the fact that Prof. Hall's original descriptions, 

 published, it should be remembered, thirty-four years ago, were 

 not always accompanied by figures of the specimens. Some of his 

 species have consequently been merged in genera and species of 

 later authors. This publication is, therefore, somewhat of a re- 

 clamation of types, and will be followed by others of a similar 

 nature giving descriptions and full illustrations from the magnificent 

 series of invertebrate types in the Hall collection exhibited in the 

 well-lighted galleries of the American Museum of Natural History 

 of New York City. A. C. 



III. — The Mean Density of the Earth. An Essay to which 

 THE Adams Prize was adjudged in 1893 in the University 

 OF Cambridge, By Prof. J. H. Poynting, D.Sc, F.E.S. 

 Pp. i. — xix. 1-156, 7 plates. (London, 1894.) 



THE subject for the Adams Prize in 1893 was " The methods of 

 determining the absolute and relative value of gravitation and 

 the mean density of the earth," and the essay which gained the 

 prize is here published in accordance with the conditions of the 

 award. Several additions have, however, been made since it was 

 first written. A bibliography containing the titles and summaries 

 of 45 memoirs has been prefixed. Part I., which is an account 

 of all the experiments so far made, has been slightly enlarged. 

 Part II. is unaltered, and is simply a reprint of the author's 

 memoir communicated to the Eoyal Society in 1891, in which he 

 describes his own determination made by means of the common 

 balance. 



In the historical discussion the geologist will find much to interest 

 him. The accounts are full but concise, and in each case end with 

 criticisms, which derive much of their value from being so often the 

 results of the author's own experience. The experiments naturally 

 fall into two classes: (1) those in which the attracting mass is a 

 portion of the earth's crust ; and (2) those which are carried on in 

 the laboratory, the attracting mass being small, easily handled, and 

 of more uniform density. With regard to the first class of experi- 

 ments, the author concludes that "our knowledge of the distribution 

 of the terrestrial matter is not yet sufficiently exact to enable us to 

 obtain good values of the mean density of the earth from the 

 observed attraction of terrestrial masses. Eather must we assume 



