lOO 



NA TURE 



[June 2, 1892 



slipshod character of some of the forms of experiment 

 recommended. The book will supply suggestions which 

 will be found useful by some teachers, but the reference 

 to apparatus unfamiliar on this side the Atlantic may be 

 a slight bar to its adoption here. W. A. T. 



OUR BOOK SHELF. 



Elementary Geography of the British Colonies. By George 

 M. Dawson, LL.D., F.R.S., and Alexander Sutherland, 

 M.A. With Illustrations. (London : Macmillan and 

 Co., 1892.) 



This volume forms one of the well-known geographical 

 series edited by Sir Archibald Geikie. The part of it for 

 which Dr. Dawson is responsible is that which deals with 

 the British possessions in North America, the West 

 Indies, and the southern part of the South .Atlantic Ocean. 

 Mr. Sutherland describes the British colonies, depen- 

 dencies, and protectorates in the northern part of the 

 South Atlantic, Mediterranean Sea, Africa, Asia (ex- 

 clusive of India and Ceylon, which are described in a 

 separate volume of the series, by Mr. H. F. Blanford), 

 Australasia, and Oceania. Both writers have enlightened 

 ideas as to the needs of those for whom such books are 

 prepared. They have carefully avoided the bringing to- 

 gether of masses of uninteresting detail, their chief object 

 being to convey a good general idea of the physical 

 features and resources of the British colonies, and of the 

 various ways in which these have affected the distribution 

 of the population and the growth of industry and com- 

 merce. The facts are presented simply and clearly, and 

 every page contains statements which an intelligent 

 teacher would have no difficulty in using as texts for 

 pie sant and profitable instruction. Most of the illus- 

 tra inns are from photographs, but there are also several 

 verv effective engravings from original drawings by Mr. 

 Pritchett. 



Farmyard Manure. By C. M. Aikman, M.A., B.Sc. 



(Edinburgh and London : Blackwood, 1892.) 

 We are told in the preface that this little work is in sub- 

 stance a chapter from a larger work on " Soils and 

 Manures," on which the author is at present engaged. 

 Perhaps we may be excused if we fail to see the necessity 

 of publishing this chapter separately in advance. It cer- 

 tainly contains much information from German works, 

 such as Heiden's " Diingerlehre," but the book is written 

 mainly from the chemist's point of view and not from the 

 farmer's. The pamphlet gives one the impression of 

 having been hurriedly prepared, but no doubt its defi- 

 ciencies will be remedied in the larger book. 



LETTERS TO THE EDITOR. 



[ Tke Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of iiPi.Tvs.n. 

 No notice is taken of anonymous communications. ] 



Peripatus from St. Vincent. 



Some of the readers of Nature will doubtless be interested 

 to learn that, while collecting in St. Vincent on behalf of the 

 Committee appointed for the investigation of the fauna and flora 

 of the Lesser Antilles, Mr. H. H. Smith obtained five examples 

 of the genus Peripatus. 



The importance of the discovery, or rather rediscovery, of this 

 Arthropod in St. Vincent rests upon the fact that the Rev. L. 

 Guilding procured the first recorded examples of the genus in 

 this same island. A description of these, under the name j'uli- 

 formis, was published by this naturalist in 1826, in vol. ii. of the 



NO. IT 79, VOL. 46I 



Zoological fournal. But from that time until now, a period of 

 66 years, no additional specimens have been brought to light in 

 this locality ; and since Guilding's types have been lost si^jht of, 

 and his description of them is wanting in detail, the identity of 

 juliformis has been involved in considerable obscurity. There 

 can, however, he little if any doubt that the examples collected 

 by Mr. H. H. Smith are specifically identical with those that 

 Guilding described. Nevertheless this assumption receives 

 more support from identity of locality tlian from the agreement 

 that obtains between the description o^ juliformis and the speci- 

 mens before me. The largest of these measures 43 mm. in length 

 and 65 in width ; the smallest, on the contrary, is only 13 mm. 

 long. One example has 34 pairs of legs, two of them 33, one 

 30, and one 29. The colour of the lower surface may be de- 

 scribed as fawn ; that of the dorsal side varies from fawn to 

 blackish grey. 



Those who are familiar with Mr. Smith's qualifications as a 

 collector need hardly be told that the specimens are on the whole 

 in a satisfactory state of preservation. I consequently hope to 

 be able to prepare a detailed description of the species, to be 

 incorporated in the report upon the Myriopoda of the Lesser 

 Antilles, the identification of the species of this group, together 

 with that of the Scorpions, Pedipalpi, and fresh-water Decapoda, 

 having been kindly intrusted to my care by the members of the 

 Exploration Committee. R. L PococK. 



Natural History Museum, May 27. 



The Line Spectra of the Elements. 



I QUITE agree with Prof Stoney that Fourier's theorem can 

 be applied to motions which approximate to non-periodic 

 motions in any assigned degree, and for any assigned time. 

 And so the co-ordinates of any arbitrary motion may approxi- 

 mately in any assigned degree and for any assigned time be 

 represented by formulas of this kind : — 



'(5^'-) 







where w^, m^, ... mn are positive integers, and; must be chosen 

 sufficiently large to suit the length of the assigned time. This is 

 not the point in Prof. Stoney's reasoning to which I object. 



What I want to say is this : If the motion is not periodical, 

 the periods of the circular functions, as well as the amplitudes 

 and phases, are not necessarily definite. That is to say, if we 

 choose a larger value of/, to get a closer approximation for a 



longer lime, the values of a, ^, a do not necessarily approach 



definite values, but may become totally different. 

 Take, for instance, the equation— 



y 



r • i 



sm- 



L ; 



i sm - -t- 



which holds good for all values of/ between -yand -f/. Prof. 

 Stoney may say that Fourier's theorem can be applied to the 

 function t. So it can, certainly, if an interval is assigned. But 

 the amplitudes and periods of the single terms are not inde- 

 pendent of the length of the interval, and do not approach definite 

 values when the interval increases indefinitely. 



The time during which the approximation is to hold good 

 need not be indefinitely long. But the time must be long 

 in comparison with the longest of the periods. Motions of 

 the ether that are represented by such functions will be resolved 

 by a diffraction grating into different rays, but others will not. 

 Prof Stoney has not noticed that a distinct property of the func- 

 tion is wanted in order to get a proper resolution into a sum of 

 circular functions. His reasonings in chapter iv. of his memoir 

 on the cause of double lines, &c. (Transactions of the Royal 

 Dublin Society, 1891), refer to all functions with or without 

 this property, and therefore do not seem to me to be correct. 

 But I admit that my expression in the passage quoted by Prof 

 Stoney might have been clearer. C. RUNGE. 



Techn. Hochschule, Hannover, May 19. 



Maxwell's Law of Distribution of Energy. 



In the current number of the Philosophical Magazine, Lord 



Kelvin describes a dynamical system in which when in stationary 



