August 28, 19 19] 



NATURE 



505 



is built, the kind of engines used to propel it 

 when cruising on the surface and when subntierged, 

 the armament, and many other details. All his 

 explanations are well illustrated. After a brief 

 discussion of the tactics of the submarine, the 

 book goes on to describe and illustrate a great 

 number of devices employed — not all successfully 

 — to track and destroy the U-boat. There are 

 many American devices the value of which has 

 probably been exaggerated — indeed, some of them 

 are merely fantastical suggestions — and these, 

 with others, are given to render the work com- 

 plete. The concluding portion of the book deals 

 with the many plans that have been put forward 

 from time to time for salving sunken ships or 

 their cargoes. Here, again, it remains for ex- 

 perience to show whether any of them are of 

 value. It is not claimed that Admiral Bravetta's 

 work can be of practical technical value, but 

 as a well written and illustrated record of the 

 achievements of human ingenuity in combating 

 a menace to the world's safety it is well worth 

 perusal by all who are able to follow semi-technical 

 Italian. In fact, an English translation might 

 well fill a want until some similar work is com- 

 piled in our own language. E. S. H. 

 Birdland's Little People: Twelve Nature Studies 



for Children. By Capt. Oliver G. Pike. 



Pp. 123. (London : The Religious Tract 



Society, 1919.) Price 4s. 6d. net. 

 The author of this volume is well known as a 

 popular writer on natural history, and presents in 

 the work before us an excellent series of essays, 

 written in an interesting style, on the habits 

 and haunts of several of the most attrac- 

 tive members of the British avifauna. The 

 subjects are well chosen, and include cer- 

 tain feathered denizens of our gardens, lanes 

 and copses, the reedy lake and the breezy 

 moorland. In each case the love-making, 

 nest-building, and subsequent care of eggs and 

 young nestlings are described graphically from 

 personal observation, so that the book is not a 

 mere compilation, but a vivid account of bird-life 

 written with the enthusiasm of a true lover of 

 feathered creatures and their entrancing ways. 

 The book will interest any boy or girl possessing 

 a fondness for animal life (and this, we fancy, j 

 includes the majority of young people), while at \ 

 the same . time the various phenomena are so i 

 accurately and carefully described that persons of | 

 mnturer years may read its pages with advantage. ; 

 The birds selected include two species of grebe, 

 two of warbler, the kingfisher, dipper, brown owl, 

 lapwing, wren, cuckoo, whitethroat, great tit, and 

 buzzard. The parasitic habits of the cuckoo, the 

 cannibalistic propensities of a young buzzard, 

 and the mysteries of migration are among the | 

 more interesting phenomena touched upon in a \ 

 book which is well printed and illustrated by a ' 

 series of twenty-four excellent reproductions of 

 photographs taken from Nature by the author 

 himself. 



NO. 2600, TJOL. 103] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 j opinions expressed by his corj'espondents. Neither 

 j 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.] 



Velocity of Electric Currents. 



While the velocity of electric waves is well known, 

 as Maxwell and Heaviside have pointed out, we 

 know absolutely nothing of the velocity with which 

 electricity travels in a wire. As Heavisi^le says 

 ("Papers," vol. ii., p. 3, line 4) :— " It may be an inch 

 an hour or it may be immensely great." 



Mr. Aston 's extremely interesting discovery 

 (Nature, June 5, p. 275), that the striae in capillary 

 tubes containing neon or helium travel with approxi- 

 mately the same velocity as that of sound in the gas, 

 is of interest in connection with the fact pointed out 

 by the writer in Science for July 22, 1892, and more 

 fully in the Physical Review for March, 1900, that 

 " resistances of equi-molecular wires of pure metals 

 are proportional to their transmission times for sound- 

 waves " (to an accuracy of about 3 per cent.), and 

 is in line with the suggestion made in the Physical 

 Review paper referred to (March, 1900), that the 

 time of travel of electricity in wires is the same as 

 that of sound. 



The double coincidence for metals and gases is at 

 least suggestive, and further work along these lines 

 might give results of interest. 



If cohesion depends on the electrons in the outer 

 rings, the tensile strength of the lead isotopes should 

 be the same; but might not their electrical resistance 

 and sound velocities (easily determined for small 

 quantities of material by resonance methods) afford a 

 means of distinguishing them ? 



Reginald A. Fessenden. 



185 Franklin Street, Boston, Mass., U.S.A., 

 August 5. 



The Magnetic Storm of August 11-12, 1919. 



The earlier months of 19 19 showed a great deal of 

 magnetic disturbance, but for some two months past 

 conditions have been unusually quiet. On the 

 morning of August 11, at about yh. G.M.T., there 

 was a "sudden commencement," followed by the 

 largest magnetic storm experienced for some years at 

 Kew Observatory. Conditions remained highly dis- 

 turbed until near loh. on August 12, when the photo- 

 graphic sheets were changed. The range in declina- 

 tion was 2° 5', and that in vertical force 9357. The 

 horizontal-force trace was twice beyond the limits of 

 registration, on each occasion for more than ten 

 minutes; thus the range shown, 8407, may have 

 been considerably exceeded. Many of the movements 

 were too rapid to be shown clearly in the trace. 

 Rapid oscillations were especially in evidence between 

 7h. and loh., and again between i4h. and i8h., on 

 August II. The declination curve also showed smaller 

 but verv rapid oscillations from midnight to gh. of 

 August "12. The extreme easterly reading, 13° 44' W., 

 was recorded at about 8h. of August 11, and the 

 extreme westerly reading, 15° 49' W., at about 

 i6h. 32m. 



The commencing movements near 7h. of August 11 



