520 



NATURE 



[August 24, 19 1( 



the crop yields consistently follow the chief ana- 

 lytical figures, and especially the ammonia. 

 Again, stable (horse) manure is said to be more 

 liable to loss on keeping- than cow manure. Recent 

 experiments show that horse manure loses much 

 less nitrogen than cow manure during storage for 

 periods of three or four months. The chapter on 

 garden remedies and insecticides is likely to be 

 very useful this summer, when pests of all kinds 

 are unusually active. E. H. R. 



The World and its Discovery. By H. B. 



Wetherill. Part i., Africa, pp. 119. Part ii., 



Asia, pp. 99, Part iii., America, pp. 131. 



Part iv. , Australia, pp. 62. (Oxford: At the 



Clarendon Press.) Price 15. each. 

 Mr, Wetherill has a story of surpassing interest 

 to tell, and he succeeds in conveying, by means of 

 the accounts of the work of the chief explorers, a 

 succinct summary of the main features of the geo- 

 graphy of the four continents other than Europe. 

 Told in this fashion, with the emphasis on the 

 lands and their peoples, the geography of the 

 remoter continents becomes vivid, and thus 

 appeals to the pupils with a sense of reality ; ex- 

 perience with this book leads to these conclusions. 

 For example, the characteristics of the people and 

 the lands near the Gambia and the Niger gain in 

 precision and definiteness in relation to the travels 

 of Mungo Park ; and the gradual development of 

 the story of the conquest of the Central Australian 

 desert provides a useful account of the control 

 exerted upon life on the earth by the absence 

 of rain in a hot region. 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for 

 opinions expressed hy 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 communications.] 



The Formation of Dust-ripples. 



Last evening when returning from a visit to the 

 trenches I noticed an interesting illustration of the 

 formation of dust-ripples. A battery of field-guns had 

 been placed nearly parallel to a road some 2000 yards 

 behind the lines. Owing to the continued fine weather 

 the roadway was covered by a coating of fine dust. 

 The guns were about 100 yards from the road, on 

 lower ground, and pointing so that the shells just 

 cleared. The battery had been in action all day. 

 There was very little wind and no traffic over the 

 road during day-time. The whole surface of the road 

 in front of the guns was covered by a series of small 

 ripples at right angles to the direction of the guns. 

 The ripples were about 1/12 in. apart, from east to 

 west. They were evidently caused by the explosive 

 wave passing over the road. The same effect can be 

 produced by discharging a Leyden jar across a spark- 

 gap near a card on which some light pov\der has been 

 sprinkled, or by tapping sharply a piece of parchment 

 stretched tightly over the end of a lamp-glass con- 

 taining fine powder. H. U. G. (C.F.). 



France, August 10. 



NO. 2443, VOL. 97] 



A Sunset Phenomenon on July 22. 



Rekekrinc; to the sunset phenomenon seen 

 July 22, and described in Nature of July 27, it se 

 probable from information kindly sent by var 

 correspondents that the clouds seen were somewl 

 in the neighbourhood of Plinlimmon. If this v 

 the case, the height of the tops of the clouds w( 

 have been from 18,000 to 18,500 ft., and the 

 clouds would have been about eight miles apart, 

 correspondent who watched the sunset from Mine 

 hampton Common reports that no clouds were vis 

 from there, but even from so far west the altil 

 of clouds at a height of 18,000 ft. over Plinlimi 

 would not have exceeded 1° 40', and they w( 

 have only been visible if the horizon were a godd 

 and the atmosphere very clear. In asking for infor 

 tion from Ireland I was casting my line too far ; the 

 of a cloud the height of which is 24,000 ft. (whic 

 probably high for a cumulo-nimbus in these latitu 

 would not be visible more than 190 miles away, 

 distance of Plinlimmon from Farnborough is 

 miles ; clouds at such distances can probably onh 

 seen when the sun sets behind them in an other 

 clear sky. C. J. P. Cav 



Meteorological Office, South Farnborough, _ 

 August 14. I 



The Utilisation of Waste Heat for Agriculture. 



With regard to Mr. Carus-Wilson's fear (Xati 

 July 27) that the heating of the earth will mull 

 pests, one may point out that earth-warming is aln 

 greatly used. Large areas of land are covered 

 glass to maintain a high temperature, and land is 

 heated directly for forcing rhubarb. One may 

 elude that farmers would welcome further means 

 heating the land if the expense were not too grea 



If the waste heat from electricity stations were 1 

 in the manner I have suggested, it would still 

 possible to remove the heat during winter month 

 destroy pests, if this were found desirable, or 

 could even cool the ground artificially. 



I would like to mention here Prince Kropotl' 

 astonishing book, " Fields, Factories, and Worksho 

 in which he shows that agriculture may be spei 

 up in a way that would surprise most people 

 look on farming as an almost non-progressive in 

 try. In it the author states that even in France, 

 its abundant sunshine, growers are experimer 

 with the direct heating of the soil, and if foum 

 advantage there, surely it would be even more s 

 this country. C. TurnbUI 



Electricity Works, Tynemouth, August 4. \ 



A Peculiar Thunderclap. 



The writer would suggest as an alternative exp' 

 tion of the peculiar thunderclap described by Mr. 

 (Nature, August 17) at different places withi: 

 circumscribed area he mentions that probabl 

 lightning discharges were not from cloud to eart 

 in the reverse direction, from a large area of i^ 

 heavily charged relieving itself at several points - 

 taneously. H. O. 



ENGINEERING EDUCATION AND 

 SEARCH IN RELATION TO 

 ORGANISATION OF BRITISH 

 GINEERING INDUSTRY. 



THE Manchester Engineers' Club, which 

 established about three years ago, int 

 among its members most of the leading engi - 

 in South-East Lancashire. During the « 

 winter of the war a series of debates was hd 



