294 



NA TURE 



[January 30, 1902 



of the theory of partial differential equations. This re- 

 striction renders necessary some originality of method 

 in problems relating to fluid motion and to the equi- 

 librium and motion of elastic solids. The book should 

 prove very useful to teachers, by showing how much of 

 these theories can be treated adequately by the aid of 

 simple analysis. Perhaps the most remarkable piece of 

 work, among those designed to make the theories of 

 mathematical physics accessible to readers whose mathe- 

 matical equipment is not very large, is the discussion of 

 the equation of transverse vibrations of a stretched 

 string ; the writer founds the theory of this equation on 

 a geometrical method, which was initiated by Riemann 

 in his memoir on the propagation of plane sound waves 

 of finite amplitude. The portion of the book dealing with 

 deformable bodies contains, among other things, a very 

 interesting account of .f//('.v.r ; the notion of stress is intro- 

 duced by means of a preliminary statement in regard to 

 the observed character of the interactions between the 

 smallest parts of bodies. Observation seems to be 

 credited here with proving things which must, from the 

 nature of the case, be remote inferences from observa- 

 tion. The subsequent deductive investigation is very 

 well done. \'iscosity in fluid_s receives a good deal of 

 attention, and the divergences between the motions and 

 resistances of perfect and of viscous fluids are illustrated 

 by comparing the two solutions of the problem of the 

 steady motion of a sphere through a fluid, regarded first as 

 perfect and then as viscous, and by the contrast between 

 the diffusion of vorticity in a viscous fluid and the per- 

 manence of vorte.x motion in a perfect fluid. Although 

 it might be wished that the treatment of the fundamental 

 theorem of rational hydrodynamics had been less sum- 

 mary, yet it will be felt that the student of theoretical 

 physics owes a debt of gratitude to Dr. \'oigt for his 

 clear outline of the theories of fluid motion. 



.\. E. H. L. 



ESSAYS ON BIRD-LIFE. 

 Birds and Man. By W. H. Hudson. Pp.317. (London: 

 Longmans, Green and Co., igoi.) Price bs. net. 



THE author of "The Naturalist in La Plata" is such 

 a close and accurate observer of nature, and has 

 such a rich store of anecdote upon which to draw, while 

 his style is so fresh and invigorating, that a hearty wel- 

 come from the public is well-nigh sure to await all the 

 efforts of his pen. In this little volume he has given us 

 a delightful series of essays dealing with bird-life in 

 England, in the course of which he dwells specially on 

 the relations between bird and man as they exist in 

 nature. Many of these essays have previously appeared 

 in various serials, but a very considerable portion of the 

 book, including the introductory chapter, is new. 



Mr. Hudson has such an enthusiastic love for bird-life 

 that, as he tells us in this introductory chapter, the sight 

 of stuffed birds in a museum is positively painful to him. 

 If this be so, an obvious and easy course lies before 

 him, and it is unnecessary on his part to say that collec- 

 tions of this nature " help no one, and their effect is con- 

 fusing and in many ways injurious to the mind, 

 especially to the young." No one, of course, wishes to 

 argue that stuffed birds are as good as living ones, but 

 NO. 1683, VOL. 65] 



since the great majority of us have neither opportunity, 

 time, patience nor money to devote to the observation of 

 birds in their native haunts, we may surely be permitted, 

 if we please, to study and admire their counterfeit pre- 

 sentments in a museum. 



Perhaps the most interesting chapter in the book is the 

 second, which bears the same title as the book itself. 

 Here the author tries to imagine what birds think of 

 man. Ki times, he thinks, they must be considerably 

 puzzled, as when a blackbird is petted while on its nest 

 by the owner of a garden, only to be shot at or stoned 

 when it leaves the protected precincts. 



" Birds" (says the author) " are able sometimes to dis- 

 criminate between protectors and persecutors, but 

 seldom very well, I should imagine; they do not view 

 the face only, but the whole form, and our frequent 

 change of dress must make it difficult for them to distin- 

 guish those they know and trust from strangers. Even 

 a dog is occasionally at fault when his master, last seen 

 in black and grey suit, reappears in straw hat and 

 flannels." 



Later on it is shown how birds clearly discriminate 

 between dangerous and harmless mammals, the author 

 giving us many interesting anecdotes derived from his 

 Argentine experiences of the relations between birds and 

 mammals in the wild state. 



.Several of the chapters — notably the one on the Dartford 

 warbler — are devoted entirely to British birds,but in others 

 the author takes a wider field. Among these latter the 

 article on geese, with its description of the vast throngs 

 of the Magellanic and upland species to be seen at cer- 

 tain seasons in .Argentina, is of especial interest. It 

 closes with a pathetic anecdote of a pair of these birds, 

 which, on account of the female having a broken wing, 

 started to walk the long journey from the pampas of 

 La Plata to distant Patagonia. 



In the two concluding chapters the author gives some 

 supplementary notes on the birds of London, and de- 

 scribes his impressions on first visiting Selborne in 1896. 

 In taking leave of this charming book we have two 

 regrets — one that it is not longer, and the other that 

 we have not space for a fuller notice. R. L. 



OUR BOOK SHELF. 

 The Earth's Beginning. By Sir Robert S. Ball, LL.D., 



F.R.S. Pp. viii -f 384. (London : Cassell and Co., 



Ltd., 1901.) Price "js. (>d. 

 This is a popular account of the nebular hypothesis, 

 based upon a course of lectures adapted to a juvenile 

 audience, and it is, therefore, almost superfluous to remark 

 that the subject is presented in simple language and that 

 no great mental effort on the part of the reader is called 

 for. The theme is one which furnishes splendid oppor- 

 tunities for the display of the powers of graphic descrip- 

 tion and illustration for which the author is so well 

 known, and the book will doubtless succeed in extending 

 the interest in this fascinating chapter of science. 



From the demonstration of the existence of true nebulic, 

 the reader is gradually led to the evidence that the sun 

 and earth once existed in nebulous form, and thence to 

 see how the present conditions of the solar system accord 

 with the hypothesis. The difficulty presented by the 

 anomalous revolutions of the satellites of Uranus and 

 Neptune is got over ingeniously by supposing that \n 

 these cases the concordant stage of the evolutionary pro- 



