November ii, 1909] 



NA TURE 



n 



fishes, and a mandible of a primitive Mastodont 

 {Tclrabclodoii dinotherioides). He is still in the field, 

 now accompanied by his sons, whom he has trained 

 10 follow him. A. S. W. 



The Book of Nature-Study. Edited by Prof. J. B. 



Farmer, F.R.S. Vol. IV. Pp. viii + 210. (London: 



The Caxton Publishing Company, n.d.) Price 



ys. 6d. net. 

 The fourth volume of this attractive publication is 

 devoted entirely to botany. The descriptions by Dr. 

 W. H. Lang of some common plants flowering in the 

 spring and summer appeared in the previous volume ; 

 the continuation refers to plants that flower later in 

 the year and others of special morphological interest, 

 -such as the honeysuckle, dodder, sundew, and types 

 of trees. The Scots pine serves as a representative 

 gymnosperm, while a brief account of pollination and 

 seed dispersal closes Dr. Lang's contributions. His 

 precise and orderly descriptions provide excellent 

 models for a student to emulate. 



To Dr. VV. Cavers has been entrusted the somewhat 

 difficult task of preparing an interesting account of 

 the cryptogams, and in this he is very successful. He 

 adopts a less formal method of description than Dr. 

 Lang, and by confining himself merely to the more 

 important characters, he is enabled to direct attention 

 to a large number of species. Most of the indigenous 

 ferns and fern allies are noted, and the more striking 

 mosses and liverworts. Lichens form the subject of 

 a separate chapter, but algae arc omitted, and only 

 outlines of the fungal groups are indicated. This 

 section and the former one are confined to morphology 

 as distinct from ecology, which provides the bulk of 

 the third section contributed by Miss C. L. Laurie, 

 although the heading, "Woodland Vegetation," 

 appears in the contents. The ecological section suffers 

 from a want of definition of the main objective. The 

 descriptions of the moors, commons, and heaths are 

 quite definite, perhaps somewhat brief, but the in- 

 terpolation of parasites and saprophytes in the middle 

 of plant associations presents an unnecessary con- 

 fusion of ideas. The chapter on woodland vegetation 

 is very instructive, albeit the paragraphs are some- 

 what disjointed, and no definite tree formations are 

 fully described as such. 



The illustrations are numerous and excellent; the 

 coloured plates of single plants, notably of the 

 honeysuckle, are admirable specimens of reproduc- 

 tion, and the plain photographs are practically 

 as effective for their purpose. Two photo- 

 •-••raphs of lichens, the woodland photographs by 

 Miss Tidman, and the illustrations of the mistletoe 

 and goat-willow are perhaps the choicest : but it is 

 somewhat invidious to draw distinctions, as the con- 

 tributors include Dr. O. V. Darbishire, Mr. H. 

 Irving, and Mr. Chalkley Gould. 



IClcments de la Tlieorie des Prohabilitds. By Emile 

 Borel. Pp. viii+190. (Paris: A. Hermann et Fils, 

 1909.) Price 6 francs. 

 Like all Prof. Borel's works, this is a very pleasant 

 liook to read. It is in three parts, dealing respectively 

 with discontinuous problems, continuous problems, and 

 those in which a priori probabilities are involved. The 

 second part contains, among other things, a useful 

 sketch of Gauss's theory of errors; and the third 

 gives some applications to statistics and biology. 

 Some of the problems are quite amusing ; for 

 instance, "Pierre plays ecart^ with a stranger, who 

 turns lip a king the first time he deals. What is the 

 probability of his being a professional cardsharper? " 

 NO. 2o8g, VOL. 82] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed 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 0/ Nature. 

 No notice is taken of anonymous communications,] 



Magnetic Storms. 



The last English mail which arrived in India brought 

 the newspapers containing the account of the magnetic 

 storm of September 25, and in the Times Sir Oliver Lodge's 

 opinion on the cause of such storms is given. It appears, 

 however, that three of the statements contained in that 

 account are not entirely supported by experimental and 

 observational evidence, and would not be subscribed to by 

 those who have recently been working at the problems of 

 atmospheric electricity. 



(i) Sir Oliver Lodge remarks: — "Some of them" 

 (electrons from the sun), " especially at the times of the 

 equinox, may come down near the equator. Those which 

 journey to the Poles are accompanied by an opposite current 

 in the crust of the earth from the equator to the Poles, and 

 this it is which disturbs the telegraphs, being picked up 

 or tapped by them en route. ^' Now if a large quantity of 

 negative electricity were suddenly added to the upper atmo- 

 sphere, and this induced a corresponding charge of positive 

 electricity on the earth's surface, it would of necessity 

 disturb the existing potential gradient in the atmosphere. 



The extent of the disturbance can be roughly calculated 

 by treating the earth's surface as an infinite plane. Sup- 

 pose that the electrons moving in the upper atmosphere 

 and adjacent space are uniformly distributed; their electro- 

 static and horizontal magnetic effect at the earth's surface 

 will then be the same as if they were confined to a plane 

 parallel to the surface of the earth. Let the surface density 

 on the plane be rr and the velocity of the ions in the 

 plane v ; H will then be changed by 2Trm<. Now Dr. Chree 

 states (Nature, September 30) that during the magnetic 

 storm H varied by more than 740 y ; for convenience let 

 us take the change in H to be only 700 7, i.e. 0007 electro- 

 magnetic unit. Hence 27r(T'J = 0-007 , ^^ fT= o-oojl 2Trv . If 

 (T is measured in electrostatic units this becomes o-oo7V/27rv, 

 where V is the velocity of light. The electrostatic field 

 between the charged plane and the earth would be i^rra, 

 i.e. 4ir xooo7V/27rD or ooi4V/i>. Expressing this field in 

 volts per metre, we have 0014 X 300 x lOoV/tJ, i.e. 420V/1;. 

 Now V cannot be greater than V, hence the smallest value 

 of the field would be 420 volts per metre. This value is 

 more than four times greater than the normal value of the 

 potential gradient in the atmosphere, and it is of the 

 opposite sign. This calculation is not supposed to be 

 quantitatively accurate, but it gives in a simple way the 

 order of magnitude of the effect. Hence it is seen that 

 the large electrical charges which would be required to 

 produce the magnetic effects observed, even if they moved 

 with the velocity of light, would be easily recognised by 

 tlieir effect on the potential gradient. For many years 

 instruments have been in use in different parts of the earth 

 giving continuous records of the potential gradient in the 

 atmosphere, but, so far as I am aware, no effect of a 

 magnetic storm has ever been reported. Such an instru- 

 ment is in use in Simla. On the day of the storm the 

 weather here was perfect, so that if any effect of the kind 

 had taken place it would have been clearly seen : but, as 

 a matter of fact, there is absolutely nothing on the record 

 to distinguish the period of the magnetic storm from the 

 periods on either side of it : the potential gradient was 

 entirely normal throughout September 25 and 26. It would 

 therefore appear that the great earth currents associated 

 with magnetic storms are not, as Sir Oliver Lodge suggests, 

 due to induced charges. 



(2) Sir Oliver "Lodge further ' says, regarding the 

 electrons : — " Those which enter the atmosphere elsewhere 

 act as nuclei for condensation of moisture, and by screen- 

 ing the sun's rays are probably responsible for some of 

 the dull and overcast weather." This statement is appar- 

 ently based on a misconception of Mr. C. T. R. Wilson's 

 experiments on the condensation of water vapour on ions, 

 for these experiments do not afford any real support to 



