January 5, 1905] 



NA TURE 



account as would warrant its use as a class text-book. 

 By means of the first five chapters a reader who knows 

 a little about the elements of electricity and magnetism 

 will be able to appreciate the nature of electric waves 

 and of Hertz's achievement in producing them. Then, 

 after briefly alluding to the early system of Marconi, 

 the writer passes on to the particular devices of Dr. 

 Braun. The book is well and clearly written, but is 

 ■in no sense a complete compendium on the subject, 

 and the reader who derives all his knowledge from 

 it will be inclined to think that there is only one system 

 in the world, and that Eichhorn is its prophet. More 

 recent methods of detecting waves by means of effects 

 arising from hysteresis in iron are dismissed in a couple 

 of pages, where there is no reference to Rutherford's 

 early detector working on the same principle, while 

 Lodge's steel-mercury-contact detector does not appear 

 even to be mentioned, although the " Literature " 

 appendix at the end includes the year 1903. In 

 appendix ii. the Thomson-Kirchhoff theory of the 

 oscillatory discharge of a condenser is given ; the 

 credit, of course, belongs to Thomson (Lord Kelvin). 



Notes on the Natural History of the Bell Rock. By 

 J. M. Campbell. Pp. xv-l-112; title-piece. (Edin- 

 burgh : David Douglas, 1904.) Price 35. 6d. net. 

 As a record of the various types of aerial and marine 

 life commonly seen by the guardians of the lonely 

 lighthouses of the east coast of Scotland in particular, 

 and of the British coasts in general, these random nates 

 are worthy of all commendation, more especially as 

 they are written by a man who does not appear to have 

 had a scientific training. Mr. Campbell was assistant 

 light-keeper on the Bell Rock for the long period of 

 nine years, and he is therefore well qualified to know 

 all that is to be known with regard to the general 

 habits of the commoner and more conspicuous species 

 frequenting the environment of his station ; while a 

 period of such a length is sufficient to include the visits 

 of many of the rarer stragglers. Most or all of the 

 notes, it appears, have been previously published in 

 the local Press of the neighbouring mainland, and they 

 are certainly worthy of rescue from such oblivion. 

 The only point for regret is, perhaps, that the author 

 does not say more about bird migration. Mr. James 

 Murdoch, late secretary to the Board of Northern 

 Lighthouses, has contributed an interesting introduc- 

 tion on lighthouses and lighthouse-men in general. 



R. L. 



The British Journal Photograpliic Almanac, 1905. 

 Edited by Thomas Bedding. Pp. 1612. (London : 

 Henry Greenwood and Co., 1904.) Price is. 6d. 

 net. 

 This bulky volume, with its mine of miscellaneous 

 photographic information, is compiled on the same 

 lines as the earlier issues, and will be found to be a 

 necessary adjunct to the studio and library. .Among 

 the host of articles in these pages may be mentioned 

 a condensed summary of the story of the British 

 Journal of Photography and the almanac which 

 appeared in the jubilee number of the above mentioned 

 journal, and also a selected number of the jubilee 

 articles. Recent novelties in apparatus, &c., by the 

 editor, forms also a conspicuous feature, and represents 

 the progress in this branch of photography. No less 

 important are the practical notes on numerous subjects, 

 the formulee, tables, list of photographic societies of 

 the United Kingdom, &c., all of which add to the 

 utility of the volume. The full indices to advertisers 

 and contents make a quick reference to any portion of 

 the book quite an easy matter, an important consider- 

 ation in a book containing 1612 pages. The processed 

 illustrations and woodcuts are as numerous as ever. 



NO. 1836, VOL. 71] 



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 with the writers of, rejected 

 manuscripts intended for this or any other part of Nature.. 

 No notice is taken of anonymous communications.] 



Mean Temperatures of High Southern Latitudes. 



On p. 131 of Nature of December 8, 1904, you give ar» 

 approximate calculation of the mean temperatures of high 

 southern latitudes, by Mr. Krebs, based upon the observ- 

 ations of the most recent Antarctic expeditions. 



For the new edition of my " Lehrbuch der Meteorologie "■ 

 I have made a similar calculation, and have made use of 

 the observations in order to calculate afresh the mean 

 temperature of the southern hemisphere. My preliminary 

 results are as follows : — • 



Mean temperature of both hemispheres : — 

 January July 



Year 



Annual 



S. hemisphere ... 173 

 N. „ ... S-o 



Whole earth ... 1 2 6 



10-3 ... 13-6 ... 70 C 

 22-5 ... 15-2 ... 14-5 „ 

 16-4 ... 14-4 . . 3-8 ,, 

 Ferrel and myself formerly determined the mean tempera- 

 ture of the southern hemisphere to be 15° C. (from tempera- 

 tures up to 55° S. lat.). The new observations in high 

 southern latitudes have now shown that the southern hemi- 

 sphere is considerably colder than the northern, viz. by 

 about i°5 C. The publication of the temperature observ- 

 ations of the Discovery's second year will be very important 

 for this question ; in my calculations I could only make use 

 of the observations relating to the first year. 



Vienna, December 30, 1904. Julius Hann. 



Reversal of Charge from Electrical Induction Machines. 



Last week, while working with a small Voss machine, 

 I accidentally observed, on stopping the machine, giving 

 about two turns in the wrong direction and then re-starting 

 the machine in the original direction, that the poles had 

 reversed. I repeated the experiment a dozen times, and 

 invariably the reversal occurred. The reversal was observed 

 by examining the spark between the knobs. 



I mentioned the fact to Prof. Gray, and we then tried 

 the effect with a vacuum discharge tube connected to the 

 knobs. While the tube was fresh the reversal occurred,, 

 but after a little time the reversal occurred but seldom. 

 It was found, however, that if the discharge was made 

 to pass by connecting one terminal of the lube to earth, the 

 other terminal to one pole of the machine, while the second 

 pole of the machine was kept insulated, then the reversal 

 invariably occurred when the procedure mentioned was- 

 followed. 



We next tried the large Wimshurst machine in the labor-- 

 atory with the same results. It was noticed, however, when, 

 the induction rods were so arranged that the machine- 

 e.xcited both ways, that the reversal did not occur. 



As I do not remember to have seen the experiment 

 mentioned before, I think it worth directing attention to, as 

 it provides a simple way of getting the discharge to pass in 

 whatever direction it is required. 



George W. Walker. 



Physical Laboratory, The University, Glasgow. 



Fishing at Night. 

 There are, as I have explained in the book referred to 

 by " S. W. " in Nature of December 29, 1904 (p. 201), many 

 reasons for night-fishing by our pilchard and other fishing 

 fleets. He quotes one, however, which is quite unsatis- 

 factory, namely, the convenience of catching the morning 



