yttnc 3, iSSo] 



NATURE 



95 



many telegraph administrations which would be bene- 

 fited by its clear practical character. But it is not im- 

 maculate. The chief defect of the book is the absence 

 of recognition of what has been done elsewhere and 

 the negation of existing literature' dealing with the 

 same subject. ]\Ir. Latimer Clark's boo'.: on " Electrical 

 Measurement" (published in iS5S) was written especially 

 for use in India. His "Electrical Tables and Formula;,'' 

 written in conjunction with Mr. Sabine and published in 

 1871, contains nearly all that is known of testing. Culley's 

 "Handbook," first published in 1866, has run through 

 six editions. Hoskiasr's " Guide to Electric Testing'' was 

 published in 1S73, snd has reached a second edition- 

 Preece and Sivewright's "Text-book of Telegraphy" 

 was published in 1S76, and has also reached a second 

 edition. Kempe's "Handbook of Electric Testing" (a 

 most useful and valuable little work) was also published 

 in 1S76. Papers by Fleeming Jenkin, Siemens, F. C. 

 Webb, Hockin, Heaviside, &c., are scattered everywhere ; 

 yet the impression left on the mind after perusing Mr. 

 Schwcndler's book is that, according to him, there is but 

 one system of testing, and that is to be found in India ; 

 and there is but one book on the subject, and here it is ! 



OUR BOOK SHELF 

 A Physical, Historical ■, IFolitical, and Descriptive Geo- 

 graphy. By Keith Johnston, F.R.G.S. Maps and 

 Illustrations. (London: Stanford, 1880.) 

 This work is in every way creditable to its unfortunate 

 young author, who, our readers may remember, succumbed 

 some months ago to the hardships of African travel while 

 leading an expedition from the West Coast towards Lake 

 Tanganyika. Mr. Johnston has not sought to enter into 

 that minute and often painful detail with which we are 

 familiar in most text-books of geography. His object has 

 been to record in each of the great departments of 

 geography the results of the latest research, leaving it to 

 the teacher or to special text-books to fill up with details. 

 After a brief sketch of some of the main points in mathe- 

 matical geography, a clear and sufficiently full sketch of 

 historical geography is given, treating not merely of the 

 progress of discovery, but of the various movements of 

 peoples and nations which have led up to the political 

 divisions of the earth as they are at present ; this we 

 think a useful introduction of scientific method into 

 history. Then follows a section on physical geography, 

 in which the most trustworthy results of research in the 

 various departments of this subject are stated with clear- 

 ness and accuracy. The remaining two-thirds of the 

 work is devoted to the special geography of the various 

 continents and countries — their physical features, natural 

 historj-, products, industries, peoples, and political and 

 social conditions. The same method is followed through- 

 out of dwelling only upon the important features. The 

 work is amply illustrated by useful and beautifully 

 executed maps, and is one of the best general handbooks 

 of geography that we know. 



Zcitschrift fiir das chemische Grossgewerhe. iv. Jahrgang. 

 Von Jul. Post. Fortgesetzt von Arthur Lehmann. 

 (BerUn : Oppenheim, 1880.) 



We have already had occasion to draw attention to the 

 merits of this publication, and the present issue of the 

 work is in no way inferior to its predecessors. It consti- 

 tutes a complete compendium of the progress of chemical 

 technology during the past year, and as such must be of 

 great service to our manufacturers. The various articles 

 are contributed by acknowledged authorities, and the 

 whole is preceded by a short review indicating the more 



striking improvements which have been introduced into 

 the chemical arts since the publication of the last issue of 

 the work. 



LETTERS TO THE EDITOR 



[The Editor does not hold himself responsible far opinions expressed 

 by his corresfondtuls. Neither can he undertake to return, or 

 to correspond with the writers of, rejected manuscripts. No 

 notice is taken of anonymous communications. 



[ The Editor urgently requests correspondents to keep their letters as 

 short as possible. The pressure on his space is so great that it 

 is impossible othei-wise to ensure the appearance even of com- 

 munications containing interesting and novel facts. \ 



The Lesser Spotted Woodpecker 



I HAVE had an opportunity lately of observing closely the 

 habits of the Lesser Spotted Woodpecker (Picus minor) as re- 

 gards t'ne very peculiar sound which it makes upon trees by the 

 action of its bill. 



It is quite certain that this habit has nothing whatever to do 

 with the quest for fo3d. The bird selects one particular spot 

 upon the trunk or boui^h of a tree, which spot is naturally 

 sonorous from the wood being more or less hollowed by de- 

 cay. The bird returns to this precise spot continually during 

 the day and produces the sound by striking the wood on the 

 spot with its bill, the stroke being repeated with a rapidity 

 which is really incomprehensible ; for it quite eludes the eye. 

 It is effected by a vibratory motion of the head ; but the 

 vibrations are so quick that the action looks like a sing'e 

 stroke. After short pauses this stroke is again and again re- 

 newed, sometimes for several minutes together. During each 

 interval the woodpecker looks round it and below it with evident 

 delight and with an apparent challenge of admiration. The 

 beautiful crimson crest is more or less erected. 



The whole performance evidently takes the place of the vernal 

 song in other birds ; and so far as I know it is the only case 

 among tlie feathered tribes in which vocal is replaced by instru- 

 mental music. 



The ne;t does not appear to be in the same tree ; but similar 

 spots are selected on several trees in the neighbourhood, and as 

 the sound is very loud and is hesrd a long way off, the hen bird 

 when sitting is serenaded from different directions. 



I have not seen or heard any attempt to vary the note pro- 

 duced by variations either in the stren Jth or in the rapidity of 

 the stroke, or by changing the point of percussion ; but I have 

 observed that the note varies more or less with the tree on 

 which it is produced. 



During about six weeks the performance has been frequent 

 every day, and early in the mornings during part of this time it 

 was almost constant. Of late it has been discontinued. In all 

 probability this is parallel to the well-known fact that singing 

 birds cease to sing after the eggs are hatched. 



This instrumental substitute for singing among the wood- 

 peckers is extremely curious. ARGYLL 



May 29 



Mr. Preston on 'Vortex Atoms 



Some passages in an article in Nature, vol. xxii. p. 56, on 

 Sir William Thomson's theory of vortex atoms, seem to show 

 that the author, Mr. Preston, has not perfectly apprehended the 

 nature of vortex motion. On p. 57 he says that "the rotating 

 portion" of the liquid "therefore glides smoothly over the in- 

 compressible liquid that surrounds it like a pipe." From this it 

 appears that the vortically-moving fluid is conceived by him as 

 slipping with reference to the rest of the fluid. This is, how- 

 ever, an incorrect view of the nature of the motion. If there be 

 an infinite mass of fluid, then the stable existence of a vortex 

 filament at any part necessitates motion throughout the whole, 

 and there is, at the surface bounding the filament, no discon- 

 tinuity of the kind apparently conceived by Mr. Preston. 



Two vortices exercise very remarkable influences on one 

 another, which are due to the irrotational motion of the parts of 

 the fluid outside the vortices. 



The existence of surfaces of finite slip in the hydrodynamics of 

 an ideal perfect fluid is not precluded by any quality attributed 

 to the fluid, but I do not think that the behaviour of vortices 

 bounded by surfaces of slipping has been hitherto treated by any 

 mathematician 



