66 



NA TURE 



[March 17, 1910 



ductor varies inversely as the distance from the 

 conductor. On p. 83, merely as a footnote, we get a 

 simplified version of Minchin's formula for the total 

 flux through a coil of circular cross-section, and on 

 p. 85 Perry's formula for the flux through a coil of 

 rectangular cross-section. On p. 87 we find the well- 

 known formula for the induction in the centre of a 

 long solenoid, but in none of these cases is a proof 

 given. 



This tendency to do without mathematical reasoning 

 is surely futile ; a reader who does not know even the 

 small amount of mathematics which suffices for the ele- 

 mentary study of electricity had better leave the subject 

 alone ; and the reader who has the required mathemati- 

 cal knowledge is not helped by finding the most simple 

 relations set forth in long tables and perfectly obvious 

 diagrams. Yet the book is interesting to the man 

 who knows the subject. He will find many things, 

 which are treated in all text-books in the orthodox 

 way, presented in a different manner, and although 

 the treatment is sometimes rather verbose, it is at 

 any rate original. As regards nomenclature, the 

 originality is perhaps carried a little too far. That the 

 term "kelvin" is used to denote the unit ordinarily 

 called the kilowatt-hour might be passed over as per- 

 missible, since some other authors have adopted the 

 same term, though it is by no" means generally 

 accepted ; but there is no justification for introducing 

 the term " Siemens" for the watt-hour. This unit is 

 hardly ever used, and to coin a special term for it is 

 quite unnecessary. The terms " continuous electri- 

 city" and "alternating electricity" are also unusual, 

 whilst the abbreviation "ats" for ampere-turns is not 

 very happily chosen. 



The first six chapters, dealing with the specific re- 

 sistance of conductors, the conception of current, 

 voltage, energy, power, and explaining Ohm 's law, are 

 very elementary. The definition of the unit of energy, 

 taken as the kilowatt-hour, is unusual. According to 

 the author's nomenclature, the "kelvin" is that 

 amount of energy which will raise the temperature of 

 a ton of water by o'86° C. This is surely a round- 

 about way for a book on electricity, especially as the 

 conception of the mechanical equivalent of heat is not 

 used to connect the "kelvin" with the "joule," but 

 the relation between the two units is simply stated in 

 a table. 



The following chapters deal with the magnetic field, 

 the E.M.F. generated in moving conductors, alternat- 

 ing currents, inductance, the magnetic circuit, and 

 insulating materials. The passage dealing with the 

 relation between time and current in a circuit to 

 which an E.M.F. is suddenly applied is an object-lesson 

 of the futility of attempting to treat such a subject 

 without mathematical basis. It cannot be done; and 

 thus we find Helmholtz's formula suddenly introduced 

 without any proof, and then worked out at great 

 length algebraically for a special case. Then we get 

 to the time constant and more numerical calculations, 

 with the usual complement of tables and curves. The 

 best chapter in the whole book is that on insulating 

 materials. Here we get on to the solid ground of 

 experimental evidence. Tables and curves are given 

 NO. 2107, VOL. 83] 



for the disruptive strength of a great variety of insulat- 

 ing materials, the influence of temperature is discussed, 

 as are also the methods of testing for disruptive 

 strength and the thickness of slot insulation found 

 necessary from practical experience. The specialist 

 who has to design high-pressure machinery will find 

 this part of the book very useful. 



GiSBERT KaPP. 



^JV ARTIST-ORNITHOLOGIST IN EGYPT. 

 Egyptian Birds, for the Most Part seen in the Nile 

 Valley. By Charles Whymper. Pp. x+221; with 

 51 coloured plates. (London : A. and C. Black, 

 1909.) Price 20s. 



WE have nothing but praise for Mr. Whymper's 

 drawings. Being more, in the nature of land- 

 scapes with birds in the foreground than figures of 

 birds with a suitable background, they naturally gain 

 from an artistic standpoint, and this has not detracted 

 from their value as guides towards identification. 

 Many of the drawings are, indeed, most pleasing pic- 

 tures, and convey a delightful impression of the 

 surroundings amongst which the visitor to Egypt may 

 expect to see the birds depicted. On the whole, Mr. 

 Whymper has not suffered greatly at the hands of the 

 block-maker, though we may remark generally that 

 the colouring of the plates is more pleasing by artificial 

 light than by daylight, and we imagine that the green 

 legs and feet of the griffon vulture, the purple hues of 

 the chats, and such like inaccuracies in colouring are 

 due to the engraver's or printer's art rather than to 

 the artist's. 



Whether the author has been wise in his choice of 

 " types " of Egyptian birds — only some fifty or sixty 

 species in all are figured and described — is perhaps a 

 matter for argument, but in a book which aims at 

 teaching the traveller in Egypt " something of the 

 birds he sees," it seems a waste of opportunity to 

 devote plates and descriptions, amongst so few, to 

 such birds as the kingfisher, house-sparrow, heron, 

 snipe, and lapwing, which every Englishman who 

 takes the smallest interest in birds must know ; 

 while the inclusion of the shoebill, which has never 

 occurred in Egypt, because it is a favourite at the 

 Giza Zoological Gardens, is really absurds-one might 

 as well include the giraffe in an account of the mammals 

 of England ! The use of the book as an aid to identifi- 

 cation is thus very small, for it will help the average 

 Englishman to identify barely forty birds which he 

 does not already know, and the volume is a large one 

 for so small a gain in knowledge. 



Turning now to the letterpress, the author disarms ? 

 minute criticism by his statement that the book is not 

 intended for the ornithologist, but he implies that its 

 purpose is, in some measure at all events, educative, 

 so that we may fairly point out some inaccuracies, for 

 we cannot commend the author's ornithology- in the 

 same spirit as we can his art. As an example, we 

 may direct attention to the description of the crested 

 lark, in which not a word is said of there being a 

 number of well-marked geographical races of this bird j 

 peculiar to different localities in Egypt — obviously s\ 

 very interesting point which, had it been explained 1 



