September 23, 1922J 



NA TURE 



407 



such work in the French or English languages, while 

 there are only two in German. He proposes a work in 

 three parts : (i.) historical introduction, (ii.) the 

 electrical field of the atmosphere, and (iii.) ions, ionisa- 

 tion, and radio-activity. Part i. has now appeared, 

 and if it may be taken as a fair sample of the whole, 

 we may expect a very welcome addition to the literature 

 of the subject. 



M. Chauveau commences his history with the 

 celebrated letter from Franklin to Collinson (dated 

 1750) in which the great American philosopher suggests 

 a method for testing the hypothesis, then fifteen years 

 ohi, that thunder and lightning are electrical pheno- 

 mena. From this commencement the following stand 

 out as the milestones along the path of progress : 



(1) The proof that thunder and lightning are elec- 

 trical phenomena : Ualibard, May 1752 ; Franklin, 

 June 1752. 



(2) The discovery of the electrification of the atmo- 

 sphere with clear skies : Lemonnier, Sept.-Oct. 1752. 



(3) The discovery of the daily variation : Beccaria, 



1 753-17 75- 



(4) The discovery of the annual variation : De 

 Saussure, 1785. 



(5) W. Thomson's (Lord Kelvin) improvements in 

 instruments and methods, and the introduction of the 

 idea of electrical potential gradient : 1856-1874. 



(6) The discovery of the conductivity of air : 

 Linss, 1887 • Elster and Geitel and C. T. R. Wilson, 

 1899. 



(7) The discover)' of " atmospheric radio-activity " : 

 Elster and Geitel, 1902. 



(8) The discovery of a very penetrating radiation 

 in the upper atmosphere : Hess and Kolhorster, 1911- 

 1914. 



In telling the story of the progress from milestone to 

 milestone, M. Chauveau has related the history of the 

 development of instruments and methods and described 

 the many attempts to find physical explanations of the 

 phenomena observed. At first the observations were 

 made with insulated conductors, generally pointed, 

 from which sparks were drawn, but Lemonnier tested 

 the electrical state of his " collector " by noting whether 

 it attracted powder, and by this relatively delicate 

 method he first detected " fine weather electricity." 

 Later rough electroscopes, fitted with pith balls or gold 

 leaves, made quantitative measurements possible, and 

 so led to the determination of the diurnal variation by 

 Beccaria. With electroscopes it was possible to detect 

 a change in the electrical state of an insulated conductor 

 as it was raised and lowered in the atmosphere (in- 

 duction effect), and De Saussure used this method with 

 remarkable results in 1785. It was with the discovery 

 of " the power of the flame " to charge a conductor 

 NO. 2760, VOL. I io] 



exposed to the atmosphere, which Volta made about 

 1780, that trustworthy methods became possible, but 

 this discovery remained practically unused, and it was 

 not until W. Thomson took up the study of atmospheric 

 electricity some seventy years later that measurements 

 were put on a sound physical basis. 



Hypotheses and theories to explain the observations 

 are innumerable : Volta's theory of the separation of 

 electricity on evaporation ; Peltier's theory of a 

 permanent negative charge on the earth's surface, 

 partially dissipated into the atmosphere by evaporating 

 water and returned on condensation ; Sohneke's theory 

 of friction between water and ice ; and Brillouin's 

 theory of the electrical separation caused by ultra-violet 

 light falling on the ice crystals of cirrus clouds ; these 

 are the most important. But every theory to explain 

 the maintenance of the earth's electrical field has failed, 

 and we appear to-day to be further from an explanation 

 of this fundamental phenomena than we have been at 

 any previous time. Even now we do not know r whether 

 the earth with its atmosphere is electrically neutral or 

 whether there is a residual charge, and at the present 

 moment there is not a single theory seriously main- 

 tained to explain the constant interchange of electricity 

 between the earth's surface and the lower atmosphere. 



G. C. S. 



Forest Policy and Management. 



(1) Schlich's Manual of Forestry. Vol. 1 : Forest 

 Policy in the British Empire. By Sir William 

 Schlich. Fourth edition, revised and enlarged. 

 Pp. xi + 342. (London: Bradbury, Agnew and 

 Co., Ltd., 1922.) 15J. net. 



(2) The Practice of Silviculture : With Particular Refer- 

 ence to its Application in the United States. By 

 Prof. R. C. Hawley. Pp. xi + 352. (Xew Vork : 

 John Wiley and Sons, Inc. ; London : Chapman 

 and Hall, Ltd., 1921.) 22s. net. 



(3) Forest Mensuration. By Prof. H. H. Chapman. 

 Pp. xxii + 553. (New York: J. Wiley and Sons, 

 Inc. ; London : Chapman and Hall, Ltd., 1921.) 

 3&y. net. 



(4) A Short Manual of Forest Management. By H. 

 Jackson. Pp. x + 70. (Cambridge: At the Univer- 

 sity Press, 1921.) 75. net. 



SIR W. SCHLICH has crowned a long life of devo- 

 tion to the science and art of forestry by the 

 publication of what is practically a new work, although 

 it purports to be only a new edition of vol. 1 of his 

 well-known Manual. (1) This volume is certainly the 

 most valuable book on the economics and the history 

 of British forestry that has yet appeared. It should 



