August 17, igi 1] 



NATURE 



guately considered, and also the advantages of the 

 Erplley omnibus when acting as a feeder for electric 

 tramwaj s. 



The latter part of the book is devoted to petrol- 

 electric vehicles and electric railways, while a chapter 

 is included which deals with electric traction curiosi- 

 ties. A full description is given of the system and 

 forking of the Metropolitan District Railway, the 

 London, Brighton, and South Coast electrified line, 

 and the " Underground " Tube combination and the 

 bther tubes. 



With regard to the petrol-electric systems, some 

 interesting facts are given dealing with its aspect with 

 regard to marine propulsion as advocated by Messrs. 

 Durtnell. Mavor, and others. The advantages of the 

 svstem, if successfullv applied to warships and liners, 

 would be enormous, but at present it has not got very 

 much bevond the experimental stages, though there 

 is hope that it may prove its worth in the near future. 



Finally, the monorail systems are described, together 

 with some other general arrangements of self-con- 

 tained generating stations on wheels, &c, making, 

 with the previous chapters, an interesting summarv of 

 the history of electric traction from its commence- 

 ment to the present day. 



British Ferns : a Pocket "Help" for the Collector. By 

 F. G. Heath. Pp. x+130. (London: Sir Isaac 

 Pitman and Sons, Ltd., iqu.) Price 2s. net. 

 The author's knowledge of fern species and their 

 habitats has been manifested in previous publications, 

 so that one is prepared to find this real pocket-book, 

 measuring 6i by 3J inches, a trustworthy and desir- 

 able acquisition when making an excursion in quest 

 of ferns. Forty-five species are enumerated, but 

 varieties with one exception are omitted. The de- 

 scriptions are written primarily for the amateur col- 

 lector, and serviceable assistance is provided in the 

 illustrations. The information is tabulated under the 

 headings frond length, description, usual habitat, and 

 localities. The list of localities, eiven as fullv as pos- 

 sible, represents an arduous piece of work. Certain 

 introductory sections are prefixed, of which the two 

 giving definitions and general habitats are most desir- 

 able and helpful, but the others are imaginative rather 

 than scientific; it is not necessarv to go bevond the 

 statement that every point of the germ (sporeling) is 

 equallv ready to produce roots or a stem. Disregard- 

 ing the first four sections, the book provides a com- 

 pact, informative guide. 



Aerial Locomotion. By E. H. Harper and A. Fer- 

 ' guson. With an introduction by Prof. G. H. 

 Bryan, F.R.S. Pp. xii+164. (Cambridge: Univer- 

 sity Press, 1911.) Price 15. net. 

 Popular handbooks on aerial navigation are now 

 Ruing from the press in a constant stream, and as 

 their number grows the reviewer naturallv judges of 

 each new arrival by comparing it with its forerunners. 

 The exact public for which the little book under 

 notice is intended is difficult to determine. The book 

 is accurate, but it is dull; it is unattractive, and is 

 poorly arranged. We cannot imagine the book being 

 read for amusement, as the style is difficult to follow, 

 or for information, owing to the absence of shoulder- 

 notes, index, sectional arrangement, and other of the 

 ' common aids to study. 



Clarity of expression is lacking in very many places, 

 while the sentence on p. 60, " If the elevator is car- 

 ried normally in a different position during flight, 

 all the conditions of flight are changed," is quite in- 

 , comprehensible. 



As has been said, the book is accurate, but it is 

 scarcelv ralcvdated to attain its apparent object of 

 interesting the public in the science of aeronautics. 

 NO. 2 l8l, VOL. 87] 



Nature's Pageant: The Story of the Seasons. By 

 Margaret Cameron. Pp. iv+ 120. (London : 

 Blackie and Son, Ltd., 1911.) Price is. 

 This little book is an attempt to teach nature-study 

 to children of seven years of age. They are supposed 

 to read the simply-worded story, in which plants and 

 animals talk, and to look at the pictures. In our 

 judgment, nature-study lessons are of little value 

 unless they are concerned with the observation of 

 the objects themselves ; and such attempts as are 

 here given to combine information with imagination 

 are not the best means of cultivating interest in 

 literature or science. 



Assaying and Metallurgical Analysis. For the Use 

 of Students, Chemists, and Assayers. By E. L. 

 Rhead and Prof. A. H. Sexton. Second edition. 

 Pp. x + 451. (London : Longmans, Green and Co.,. 

 1911.) Price 125. 6d. net. 

 The differences between this edition and the preceding 

 one are not important. A few new methods are in- 

 cluded, such as the determinations of copper and of 

 iron by titanous chloride and" the volumetric estima- 

 tion of nickel by cyanide, but the text generally re- 

 mains unchanged, and the merits and occasional 

 defects of the book have not been modified. It is still 

 one of the most useful works on the subject available. 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond wiih the writers of, refected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



On the u-Ray Theory of Aurora Borealis. 

 In a letter to Nature of April 13 (vol. lxxxvi., p. 213) 

 I gave reasons for the view that at least some of the forms 

 of aurora borealis are caused by a type of rays which, as 

 regards absorption by matter, follow the same law as 

 a rays from radio-active substances. In the letter I also 

 mentioned that the diurnal distribution of aurora appar- 

 ently would require a negative charge of the rays. 



My arguments which lead to a negative charge was 

 based on the assumption that the simplest orbits, like 

 that of (a), ought to occur more frequently than the more 

 complicated, like that of (b), both of which are theoretic- 

 ally possible. This assumption, which indeed might seem 

 legitimate, is not, however, a consequence of exact mathe- 

 matical calculation, for the problem of finding the relative 

 probability of the occurrence of the various possible orbits 

 has not yet been solved. 



Further, an exact determination of the diurnal distribu- 

 tion of aurora is made difficult through the effect of sun — 

 and moonlight; but, even if we take it for granted that 

 the aurora are most frequently' found on the evening side, 

 there is, so far as our present knowledge goes, no 

 necessity for assuming a negative charge of the cosmic 

 rays. 



Moreover, the explanation of the- thin drapery form, 

 given bv Stormer. 1 requires orbits like (/>) having turned 

 round the magnetic axis a great angle, and if such an 

 orbit is going to strike on the evening side a positive 

 charge is necessary. These matters will be more fully 

 discussed in a subsequent paper. 



There are some other points not mentioned in my 

 previous note which are of considerable interest. In order 

 to explain from the radiation theory the formation of thin 

 drapery bands, a strictly homogeneous radiation is neces- 



> Arch, de; S c :. phys. tl N'at., 1907. 



