S56 



NATURE 



[Oct. 13, 1 88 1 



being of interest in itself, also has reference to the impor- 

 tant subject of denudation :— 



" Little horizontal led;_;es, one above another, have been 

 observed on steep grassy slopes in many parts of the 

 world. Their formation has been attributed to animals 

 travelling repeatedly along the slope in the same horizon- 

 tal lines while grazing, and that they do thus move and 

 use the ledges is certain ; but Prof Henslow (a most 

 careful observer) told Sir J. D. Hooker that he was 

 convinced that this was not the sole cause of their 

 formation." 



It is then shown that the initial cause of these ledges 

 is the burrowing of earthworms. For, 



" If the little embankments above the Corniche Road, 

 which Dr. King saw in the act of formation by the accu- 

 midation of disintegrated and rolled worm-castings, were 

 to become confluent along horizontal lines, ledges would 

 be formed. Eich embankment would tend to extend 

 laterally by the lateral extension of the arrested castings ; 

 and animals grazing on a steep slope would almost cer- 

 tainly make use of every prominence at nearly the same 

 level, and would indent the turf between them ; and 

 such intermediate indentations would again arrest the 

 castings." 



Thus, on the whole, it will be seen how important an 

 agency in nature Mr. Darwin has shown the action of 

 worms to be, so that, in his own concluding words, " it 

 may be doubted whether there are many other animals 

 which have played so important part in the history of 

 the world as have these lowly organised creatures.'' 



George J. RoM.-iXF.s 



OUR BOOK SHELF 



The Atlas-Geography. By A. H. Macdonell. (London: 

 H. K. Lewis, 188 1.) 



Under this title Mrs. Macdonell has attempted tosupf ly 

 what she believes to be a want long felt in teaching 

 geography to young children. She finds, as every teacher 

 finds, that children prefer the map to the book, and so 

 she provides the means of teaching geography by means 

 of an atlas. The Atlas-Geography consists of nine 

 double maps. First we have in each case a coloured 

 map with the leading names filled in, and facing it a list of 

 the leading features in the map, countries, their divisions, 

 towns, oceans, islands, capes, rivers, &c., which the chil- 

 dren learn by heart, fixing at the same time their positions 

 on the maps. Following this is a corresponding uncoloured 

 map, without names, on which the children should be 

 able to point out the features without assistance. Facing 

 this is an interesting and simple descriptive account of 

 the leading characteristics of the continent or country 

 to which the map refers. It will thus be seen that in the 

 hands of a painstaking and judicious parent or teacher 

 the Atlas-Geography ought to prove a most valuable help 

 in interesting children in the subject, and in enabling them 

 to acquire the leading facts. The maps are well executed, 

 clear, and not over-crowded ; they are the World, 

 Europe, Asia, Africa, Australia, North America, South 

 America, the British Isles, and Palestine. 



Gesammelte Abhandlungcn vnd kleinere Schtifteii zur 



Pflanzengeographie. The collected treatises and shorter 



writings on Phytogeography of the late A. Grisebach, 



edited by his son, Dr. Ed. Grisebach. 8vo, pp. 628. 



(Leipzig : Wilhelm Engelmann.) 



As the editor states in his preface, the present volume 



combines for the first time the numerous writings on 



phytogeography of the late Prof. A. Grisebach, spread over 



a period of thirty years, and scattered in various journals 



and publications, several of them very difficult of access. 



Constant reference is made to many of these writings in 

 the " Vegetation der Erde"(iS72); hence their publica- 

 tion in a collected form is a great boon. In addition to 

 those article published previous to the " Vegetation der 

 Erde," this volume contains the author's subsequent 

 reports (1S66-76) on the progress in the geography of 

 plants. It also contains a biographical sketch of the late 

 Prof. Grisebach, together with the bibliography of his 

 works, .^n excellent French translation of the "Vege- 

 tation der Erde" appeared in 1874, but no English edi- 

 tion has been published, nor would we recommend the 

 publication of one now, because the data that have been 

 accumulating during the last decade would justify the 

 publication of an original work, treating the subject from 

 a different standpoint. W. B. H. 



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 'vith the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. 



[ The Editor urgently requests correspondents to keep their letters 

 as short as possible. The pressure on his space is so great 

 that it is impossible otherwise to ensure the appearance ev^n 

 of communications containing interesting and ncvel facts.l 



The Solar Outburst of July 25, 1881 



In the intere.sting acconnt of .a solar outburst on July 25 

 contained in your la.5t number (p. 50S), Mr. Hennessey says 

 that "unhappily the sun remained invisible till July 30." 

 Referring to our sketche,'; of the polar >urface, I find that the 

 nearest in time to the date of the outburst are those made 

 on July 21 and 27. On the inlervening diiys clouds pre- 

 vented all solar work. The sketcb on July 21 shows the groups 

 in the [«/] quarter of Mr. Hennessey'^ disk, and that of July 27 

 gives those in the [«/] and [j-/] portions, and al o ihe two 

 gr./ups in the [«/] which were farthest from the centre on the 

 2ist. There was certainly not the slightest trace on the 21st of 

 the remarkable group which burst forth so suddenly on the 25th, 

 and there can be very little d'.ubt that the spots in the \,n p\ 

 quarter on the 27lh are identically the same as those in the [«/] 

 quarter on the 21st. Drawings of the solar disk are made here 

 on every available day, and the position of each spot is marked 

 with die greatest exactness ; but when the ^ky is cloudy, as on 

 the 27th, it is not always possible to fill in all 'he details. The 

 exact position cf each spot is invariably marked before any 

 details are sketched, and therefore, as the definition on the 27th 

 was good, the group, which suddenly appeared near the centre 

 of the di^k on the 25th, must already have completely vani.shed. 

 I might mention, in conclusion, that our magnetic photograms 

 show no sign of any distm-bance synchronous with the solar 

 outburst. S. J. Perry 



Stonyhurst Observatory, Whalley, .September 30 



On the Velocity of Light 



With reference to Lord Rayleigh's article on the Velocity of 

 Light (vol. xxiv. p. 382) I, and possibly others, find it difficult 

 to follow him when he s.ays, in the case of A\ the methods for 

 determination of the velocity of light except the aberration 

 method, that the velocity arrived at is the "group velocity, " and 

 not necessarily the "wave velocity." I, for one, should be gkad 

 of further exposition. Does not Foucault's revolving mirror 

 experiment, for instance, meanire the velocity of motion of the 

 centre of the di-turbance which is transmitted from mirror to 

 min-or? And would it not be the case that, if the naves moved 

 faster than the groups, ne.v groups would be continually formed 

 ahead, the old ones dropping out behind : so that the centre of 

 the disturbance would not remain in any given group? Further, 

 is any credence to be given to the result that blue light travels 

 anything like I '8 faster than red light, while this is unconfirmed 

 by the colours of Jupiter's sateUites ? W. H. Macaul.'W 



Mountsorrel, August 29 



An Aquatic Hymenopterous Insect 

 The following circumstance may prove interesting, and pro- 

 bably new, to some of your entomological readers. OnSeptem- 



