April II. 1889] 



NATURE 



557 



I 



he has collected from the published records of various 

 travellers, such as Livingstone, Serpa Pinto, Fritsch, and 

 others. 



He limits the area of his discussion by biological con- 

 siderations, as he defines the extreme southern limit of 

 tropical Africa to be that fixed by the cultivation of the 

 date-palm and the existence of the tsetse fly. 



We have said that the records of observations are 

 somewhat scanty, and this remark will be justified when 

 we point out that from many stations the results for two 

 years, or even less, are printed. The figures, such as they 

 are, have been, however, most conscientiously discussed. 



The area is divided into four great districts, classified 

 according to the period of occurrence of the rainy season, 

 viz. (i) the winter rains, (2) the intermediate region of 

 spring -and autumn rains, (3) the heavy summer rains, 

 (4) the West Coast. Under (i), as subdivisions, we have 

 the South-West Province, the Western Karroo, and the 

 Little Namaqua Land. Under (2), the South Coast, South 

 Karroo, North Karroo, and the South-East Mountain 

 Land. Under (3), the Table-land of the Upper Orange 

 River, the North Transvaal, the Kalahari, and the Great 

 Namaqua and Damara Land. 



After treating of these several regions at considerable 

 length, Dr. Dove proceeds to discuss the possible develop- 

 ment of agriculture in the different districts. His panacea 

 for the Kalahari and some other tracts, with pure sandy 

 surface, in the northern part of the area, is to introduce 

 the date-palm. 



He concludes the work with a discussion of the rainfall 

 and its distribution, with some remarks on the question 

 of the alleged deterioration of the climate by the drying 

 up of the country. This effect he considers, with Mr. 

 Gamble, to be merely the outcome of reckless forest 

 destruction. 



He points out the brilliant results obtained, at com- 

 paratively small cost, by the construction of reservoirs, as 

 at Beaufort and at Van Wyli's Vley. R. H. S. 



Chambers s Encyclopedia. New Edition. Vol. IIL 

 (London and Edinburgh : W. and R. Chambers, 1889.). 

 It may be enough to say of the third volume of the new 

 edition of " Chambers's Encyclopaedia " that it falls in no 

 respect below the high level maintained in the preceding 

 volumes. The editor is working upon a well-conceived 

 plan, and he has every reason to be satisfied with the 

 manner in which individual subjects are dealt with by his 

 contributors. Scientific subjects continue to receive the 

 attention which properly belongs to them in such a work 

 as this. Tbe treatment of coal, coral islands, and geology 

 generally has been intrusted to Prof James Geikie, 

 and his articles are admirable examples of compact and 

 lucid exposition. Mr. J. Arthur Thomson writes of cater- 

 pillars, cells, crabs, «S:c. ; Dr. Leonard Dobbin, of 

 chemistry ; Dr. Alexander Buchan, of climate ; Mr. R. T. 

 Omond, of clouds ; and Dr. R. A. Lundie, of colour- 

 blindness. Mr. C. J. Woodward has an article on crystal- 

 lography, and Dr. W. Peddie treats of dew and diffusion. 

 Of the articles on Darwin and the Darwinian theory, the 

 former is contributed by Mr. Grant Allen, the latter by 

 Prof Patrick Geddes. There are a good many geo- 

 graphical articles, among which we may especially note 

 the article on China, by Prof. Legge ; that on the Congo, 

 by Sir Francis de Winton ; and that on Constantinople, 

 by Mr. Stanley Lane-Poole. So far as we have been able 

 to test the various papers, we have found them carefully 

 written and thoroughly trustworthy. 



The Elementary Principles of Electric Lighting. 



By A. A. C. Svvinton. Second Edition. (London : 



Lockwood.) 

 The author explains generally the different apparatus 

 used in electric lighting, and the broad princples of their 



working, using the "water- works" theory of the electric 

 current, but at the same time carefully explaining that 

 this is only done for the sake of convenience. An un- 

 fortunate mistake has been made in the diagram of the 

 continuous-current dynamo (p. 24), where the coils are 

 shown as wound in a different sense on the two limbs of 

 the field-magnet. The book is, however, a remarkably 

 clear exposition of the subject, and at the same time a 

 model of conciseness. 



The Natural History and Epidemiology of Cholera. By 



Sir J. Fayrer. (London: Churchill, 1888.) 

 The above formed the subject of the annual oration de- 

 livered by Sir Joseph Fayrer before the Fellows of the 

 Medical Society. 



The author deals at length in a most interesting way 

 with the history of the disease, and then proceeds to en- 

 large upon its geographical distribution, habits, conditions, 

 and epidemic movement. The aetiology of cholera is 

 then dealt with, together with a review of those general 

 and special precautionary measures it is desirable to 

 adopt. 



Throughout, the essay is written in a clear and interest- 

 ing manner, and from the vast experience of the author 

 in the subject the oration will well repay a careful 

 perusal. William Robert Smith. 



LETTERS TO THE EDLTOR. 



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

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond 7vith the writers of, rejected 

 manuscripts intended for this or any other part of NATURE. 

 No notice is taken of anonymous communications. ] 



Halo and Mock Suns. 



This morning a very distinct halo accompanied by mock suns 

 on either side was seen here. As the latter, owing to the altitude 

 of the sun, were at a considerable distance outside the halo, I 

 think the following details are worth putting on record. At 

 iih. 12m., Berne time, the sun's altitude being 48° 30', the 

 distance from the halo to the left-hand mock sun was 9° 15'. 

 The parhelic circle was plainly visible, reaching from the sun 

 slightly beyond the mock suns. Each of the latter consisted of a' 

 reddish patch shading off into white and then into blue on the 

 side away from the sun. From the brightest red to the brightest 

 blue was about 2°, and the measurement g" 15' was taken from 

 half-way between these to the nearest point of the circle dividing 

 the red from the blue of the halo. It is difficult or impossible to 

 measure such faint objects with the sextant. So I held a pencil 

 at both arms' length, and noted the length on the pencil corre- 

 sponding to the desired angle. Holding the pencil with both 

 hands gives it a very definite distance from the eye, provided 

 the position of the body and the altitude of the object be not 

 much altered. Paying attention to these points I measured the 

 angle subtended at my eye by a certain length on the side of a 

 house, both with the pencil and a sextant. The angle g" 15' was 

 found thence by simple proportion. I think the error of this 

 measurement can hardly exceed 30'. The halo of course was the 

 common one of 22°. James C. McConnel. 



Davos Platz, April 5. 



On the Connection between Earth Currents and Changes 

 in Solar Activity. 



May it not be that, in the recent experiments of Mr. Hertz on 

 the effect of ultra-violet light on electric disch-arge, we have an 

 explanation of the relation existing between disturbances on the 

 solar surface and disturbances in earth currents ? 



The evidence for such a connection is obtained from the 

 Greenwich records. 



If we make the not very violent assumption that two clouds 

 differ in potential from each other and from the earth, it will be 

 seen thjit the earth will act as a condenser, and underneath each 

 cloud will be collected a charge of opposite sign. 



With sunlight, Hertz failed to find any marked effect, prob- 



