54 = 



NA TURE 



[OCTOUEK 4. 1894 



generated in the sky by a fulgureous exhalation con- I 

 globed in a cloud by the circumfixed humour, and are as it 

 were baked hard by intense heat, and the weapon becomes 

 pointed by the damp mixed with it flying from the dry 

 part, and leaving the other end denser, but the exhalations 

 press it so hard that it breaks out through the cloud, 

 and makes thunder and lightning. But, he says, if 

 this be really the way in which they are generated, it is 

 odd that they are not round, and that they have holes 

 through them, and those holes not equal through, but 

 widest at the ends. It is hardly to be believed he 

 thinks." ' 



Here we have an example of a brilliant and compre- 

 hensive theory — a theory able to explain everything, 

 yet subject to petty criticism ! .And we fear that our 

 anonj-mous author's equally brilliant theory of the orgin 

 of the whale will be not less unfortunate. Of course we 

 are assured that the theory explains almost everything — 

 homology, embryology, rudimentary organs, &c., though 

 he does, modestly, admit that it does tiot explain why 

 hybrids are sterile. In order not to misrepresent the 

 writer one more passage must be quoted, because he 

 there brings his ideas more nearly into accord with that 

 theory of discontinuous variation which has been recently 

 put forward. 



" Evolution proceeded by successive distinct gradations 

 or stages. The differentiation of every new species 

 resulted from forces ah extra superimposed on, and, to 

 some extent, superseding or modifying the forces that 

 produced the species or genus immediately preceding in 

 the same line of development. The fecundated ovum 

 cf a species was, as it were, fecundated a second time 

 with a new force, and the ovum thus bi-fecundated pro- 

 duced, instead of the species to which it belonged, a new 

 species built upon a modification of its predecessor." 



The theory is therefore one of special creation through 

 the ordinary process of descent. The " new forces ah 

 i.vtra" which produced a whale from a terrestrial animal 

 were also at work every time one species of tit, or 

 warbler, or beetle, or snail, was modified in adaptation to 

 a slightly difl'erent mode of life, and became a new 

 species. Thus all is explained ; except why there is any 

 variation of these specially adapted species, why they in- 

 crease at such an enormous rate necessitating such 

 wholesale destruction, why there is any struggle for 

 existence. All these phenomena, which are the very 

 essence of a theory of descent with modification by 

 natural selection, are entirely out of place in a theory of 

 special creation, and are therefore the condemnation of 

 any such theories. Alfred R. Wallace. 



THE MEAN DENSITY OF THE EARTH. 

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

 A(Ltms Prize was a,ljud};ed in 1 893 in the University 

 of Cambridge. By J. H. Poynting, Sc.D., F.R.S. 

 (London: C. Grififin and Co., Limited, 1894.) 



Tni.S essay, which contains an account of Prof. 

 Poynting's well-known investigation of the mean 

 density of the earth, though the last Adams prize essay, 

 IS the first to which that prize has been awarded for 

 experimental work. We hope that it is the first of a 

 long series of essays in which the candidates will attack 



1 " Ear'y lliftory of .Mankind," fccond ciition, p. 337. 



NO. 1301, VOL. 50] 



the questions proposed by experiment as well as by 

 mathematical analysis. We can hardly expect, however, 

 that the level reached by the magnificent experimental 

 work of Prof. Poynting will always be maintained. 



The essay consists of two parts, the first containing an 

 account of previous determinations of the mean densitv, 

 the second an account of Prof. Poynting's own deter- 

 mination by means of the ordinary balance. 



The first part begins with an account of the astro- 

 nomical or geodetical methods, in which the attraction 

 of a mo\mtain was compared with that of the earth, as in 

 the experiments of Bouguer in Peru, of Maskelyne and 

 Hutton on Schehallien, of James and Clark on Andrews 

 Seat, of Carlini on Mount Cenis, and of Mendenhall on 

 Fujiyama ; or with that of the slab of matter above the 

 surface of a mine as in Airy's Harton Pit experiments, 

 and von Sterneck's experiments in Pribram and Freiberg 

 The beautiful method employed by von .Sterneck in his 

 pendulum experiments ought to be more widely known 

 in Kngland. The object of the astronomical method has 

 undergone a curious reversal. It was originally to deduce 

 the mass of the earth from a supposed knowledge of the 

 distribution of matter in the locality of the experiment, 

 whereas now it is rather to find the distribution of mattei 

 in this locality, assuming the mass of the earth to be 

 known. 



The other methods are laboratory methods, and 

 depend upon the measurement of the attraction be- 

 tween known masses. Prof. Poyntmg points out a vei v 

 interesting under-estimate of this attraction made bv 

 Newton. In the Principia, Xewton estimated that twn 

 spheres of the density of the earth, each a foot in dlametei . 

 would, if separated by quarter of an inch and left t ' 

 their own attractions, take nearly a month to come into 

 contact. Prof. Poynting shows th.it there is a mistake in 

 the arithmetic, and that in reality the spheres would 

 come into contact in between five and six minutes. 



It is now very nearly a century since the first measure- 

 ments of the attraction between two masses in a labora- 

 tory were published by Cavendish ("Experiments t 

 Determine the Density of the Earth," /'////. Trans, ijg^^ . 

 who used the torsion balance. Since then this method 

 has been used by Reich, Haily, Cornu and Bailie, and 

 Boys ; while the ordinary balance has been used by von 

 Jolly, Prof. Poynting himself, and by Konig, Kichari 

 and Krigar Menzel, working in collaboration, while the 

 method of the pendulum balance has been used by 

 I Wilsing. The labour expended over these investigations 

 I may be estimated from the fact that, to take only two 

 I modern instances. Prof. Poynting's experiments extended 

 1 over twelve years, while those of Cornu and Bailie 

 I were commenced in 1870, and are not yet completed. 

 The essay contains a clear and critical account of the 

 preceding experiments. The result of the criticism is 1 

 raise, if possible, Cavendish's fame as an experimentci 

 Of Baily's laborious research, Prof. Poynting says : 

 "The critical examination it has received in later yeai^ 

 has entirely destroyed any confidence in the result. I 

 remains, however, as a most remarkable and uselui 

 ex.ample of the danger of substituting multiplication of 

 observations for consistency. ' The contrast between the 

 amount of work which has been published on the 

 j numerical magnitude of the attraction, with tliat wliic 



