172 



NATURE ^ 



[June 19, 1890 



the apparatus required, then treats of the Algae as a 

 class, and the main divisions into which they have been 

 separated by botanists, and in most of the remaining part 

 of the book describes species, " choosing as types of each 

 genus such species as are most likely to be met with, and 

 leaving out those which are either rare or possess few 

 points of interest for the beginner." Mr. Smithson him- 

 self points out that the volume leaves much to be sought 

 elsewhere ; but, if used intelligently, it will do sound work 

 by preparing the way for wider study. 



Rambles and Reveries of a Naturalist. By the Rev. 



William Spiers, M.A., F.G.S., &c. (London : Charles 



H. Kelly, 1890.) 

 Mr. Spiers does not profess to give in this little book a 

 full account of any one of the subjects with which he 

 deals. His aim has been " to awaken or to stimulate a 

 love for Nature in the minds of some who may not as 

 yet have suspected what wondrous and ever-varying 

 beauty lies everywhere about us, in ditch and pond, in 

 rock and stone, in river and sea, on earth and in the 

 skies." With this end in view, he describes, in a series 

 of short sketches, various phenomena which he himself 

 has had opportunities of observing ; and he does his 

 work so well that to a good many readers his book may 

 be of considerable service. There is nothing new or 

 brilliant in Mr. Spiers's descriptions ; but they are fresh 

 and clear, and display not only a genuine love for Nature, 

 but a capacity for appreciating the scientific significance 

 of many different orders of facts. Besides other essays, 

 the volume includes papers on seaweeds, rambles in 

 Cornwall, a visit to the Channel Tunnel, St. Hilda's 

 snake-stones, tiny rock-builders, and an evening at the 

 microscope. 



Sketches of British Sporting Fishes. By John Watson. 



(London : Chapman and Hall, 1890.) 

 A PREFATORY note to the " Sketches " tells us that " the 

 subject-matter has, for the most part, been gleaned 

 directly from the waterside, and should be looked upon 

 more as the notes of a naturalist than the jottings of an 

 angler." Accordingly, it was with anticipation of interest 

 that we turned to the opening chapter, on salmon. 



So little is known of the natural history of the salmon, 

 and so great is its value, both for sport and for food, that 

 we eagerly scan the pages of a naturalist and an angler 

 who may tell us what he has seen and knows. Mr. 

 Watson has nothing to tell us. He disposes of the 

 salmon in 12 pages, and the impression produced upon 

 us is that his acquaintance with that noble fish is confined 

 to the fishmonger's slab, and to the dinner-table. 



The chapter on trout is little more satisfactory. That 

 on grayling is by another hand. 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed 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 commtcnications. ] 



Coral Reefs, Fossil and Recent. 



Dr. von Lendenfeld has (June 12, p. 148) quoted cases to 

 contest my statement that there are no coral reefs whose slopes 

 are known to descend steeply to greater depths than about 

 4000 feet. I must take these seriatim. 



(i) "Fitzroy's no-bottom sounding of 7200 feet at a distance 

 of 6600 feet from the breakers at Keeling Island." 



I hope I shall not be misunderstood when I say that I cannot 

 accept this as conclusive evidence. Experience daily shows us 

 how little confidence can be placed in a single deep sounding, 

 taken before the days of suitable apparatus, and with no descrip- 



tion of the means employed, either to fix the position exactly, or 

 to obtain the cast. It may be correct, but on the other hand it 

 may not be. 



(2) "Maldives, &c., rise from a bank of 1000 fathoms very 

 abruptly." 



I cannot find any deep soundings near these groups at all. 

 One sounding of 1243 fathoms at a distance of 10 miles is the 

 closest. 



(3) "Bermudas rise abruptly out of a depth of 12,000 to 

 13,000 feet." 



There is only one sounding of 12,000 feet anywhere near 

 Bermuda, and as that is six miles from the nearest shallow 

 water, the isolated Challenger Bank, it represents a slope of 

 only 19°. 



In point of fact, very few slopes of coral formations have yet 

 been accurately measured. Among the most remarkable that I 

 know are : — 



Bougainville Reef in Coral Sea, which drops perpendicularly 

 from the water-level to 360 feet ; at a mean slope of 76° to 780 

 feet ; and at 53° to 1500 feet. 



Dart Reef, in same sea, has a mean slope of 64° to 1200 feet. 



Macclesfield Bank, a so-called," drowned" atoll, in China 

 Sea, has a mean slope of 51° to 4200 feet, and possibly more. 



The existing conditions of the steep outer slopes of atolls are 

 sufficiently astonishing. All I wish to maintain is that we 

 should argue upon proven facts, and not assumptions, which 

 tend to exaggerate difficulties, and to lead us astray. 



With regard to Dr. von Lendenfeld's explanation of the 

 limitation of depths of lagoons, I must await a better before 

 I am convinced. My point is that it is very remarkable that no 

 matter how vast the lagoon, and how deep the steep outer 

 slopes, no lagoon has more than a certain depth, and that such 

 a limited depth that isolated coral heads can spring out of it ; 

 and I cannot make this general fact fit with a general theory 

 of subsidence, even when varied by occasional elevations. 



The " drowned " atolls are no deeper than others whose rims 

 are at the surface ; vide Great Chagos Bank, and Suadiva Atoll 

 in Maldives. W. J. L. Wharton. 



June 14. 



NO. 1077, VOL. 42] 



ELECTRO-MAGNETIC RADIATION} 



IN order to discover whether actions are propagated in 

 time or instantaneously, we may employ the prin- 

 ciple of interference to measure the wave-length of a 

 periodic disturbance, and determine whether it is finite 

 or no. This is the principle employed by Hertz to prove 

 experimentally Maxwell's theory as to the rate of propa- 

 gation of electro-magnetic waves. In order to confine 

 the experiments within reasonable limits we require short 

 waves, of a few metres' length at most. As the highest 

 audible note gives waves of five or six miles long, and 

 our eyes are sensitive only to unmanageably short waves, 

 it is necessary to generate and observe waves whose fre- 

 quency is intermediate between them, of some hundred 

 million vibrations per second or so. For this purpose we 

 may use a pair of conducting surfaces connected by a 

 shorter or longer wire, in which is interposed a spark-gap 

 of some few millimetres' length. When the conductors 

 are charged by a coil or electrical machine to a suf- 

 ficiently high difterence of potential for a spark to be 

 formed between them, they discharge in a series of oscil- 

 lations, whose period for systems of similar shape is 

 inversely proportional to the linear dimensions of the 

 system so long as the surrounding medium is unaltered. 

 When the surrounding non-conducting medium changes, 

 the period depends on the electric and magnetic specific 

 inductive capacities of this medium. Two such systems 

 were shown : a large one, whose frequency was about 

 60 millions per second ; and a small one, whose frequency 

 was about 500 millions per second. The large one con- 

 sisted of two flat plates, about 30 cm. square and 60 cm. 

 apart, and arranged in the same way as is described by 

 Prof. Hertz in Wiedemann's Antialen, April 1888. The 



' Friday Evening Lecture delivered at the Royal Institution, on March 

 21, by Prof. G. F. Fitzgerald, F.R.S. 



