500 



NATURE 



\Oct. 2 2, 1874 



the trot, gallop, &c. The rise in the trot is sudden and 

 simultaneous with the time the animal's feet are on the 

 ground, and the fall with the time of suspension. In the 

 gallop the same is the case, though the rises and falls are 

 less sudden ; they are, " therefore, less jarring to the rider, 

 though they may, in fact, present a greater amplitude." 

 ( To be continued^ 



OUR BOOK SHELF 



I. — Les Roches ; Descriptions de leurs Elements: Methode 

 de Determination, etc. Par Edouard Jannetaz, Docteur es 

 Sciences, etc. (Paris : J. Rothschild, 1874) 

 2. —Les Minerau.v : Guide Pratique pour leiir DSlermi- 

 nation, etc. Par F. de Kobell. Avant-propos et Addi- 

 tions, pir F. Pisani, Professeur de Chimie et de Mini^ra- 

 logie. (Same publisher.) 

 3. — Le Monde Microscopiqne des Eau.r. Par Jules 



Girard. (Same publisher.) 

 These three works form part of a series of popular 

 scientific treatises issued by the enterprising Paris pub- 

 lisher, M. Rothschild. They are small volumes neatly 

 printed and got up, and Nos. i and 3 are fully illustrated 

 with well-executed cuts. 



No. 1 is intended as a practical guide for the use of 

 engineers, geologists, mineralogists, agriculturists, and 

 pupils of Governtnent schools. It is illustrated with 

 ihirly-nine woodcuts, contains a great deal of valuable 

 i.iformalion in small space, and seems well calculated to 

 form a useful little handbook for the classes mentioned. 



No. 2, which is a translation from the tenth German 

 edition of Kobell's work by Cojut L. de la Tour-du- 

 Pin, with a preface and additions by Prof. Pisani, is 

 intended for the use of chemists, engineers, manufac- 

 turers, &c., and, like the above, seems well calculated to 

 serve its purpose, of helping those who have a moderate 

 knowledge of cUemistry to analyse speedily and exactly 

 the principal minerals. 



No. 3 is of a much more popular kind than the two 

 previously mentioned works ; its author, I\I. Girard, is 

 «ell known as a success'^ul populariser of scientific results. 

 It contains sixty-ei^ht beauli ul and useful cuts* It is 

 intended as a handbook to those who wish to derive 

 amusement and instruction from the use of the microscope, 

 and takes up successively sjme of the principal points in 

 the animal, vegetable (existing and fossil), and mineral 

 kingdoms. 



Nos. I and 2 have very full indexes appended. 



LETTERS TO THE EDITOR 



[ T/ie Editor docs not hold lumself responsible for opinions expressed 

 by /lis correspondents. Neither can he tinder take to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous coiiimuiiications.'\ 



Periodicity of Auroras 

 On my return to Newcastle-on-Tyne I take the opportunity 

 of being able to recur to books of refcrmce to reply to a question 

 put by Mr. Procter, in N.4TURE (vol. x. p. 355), whttlier any 

 complete cataloguei of auroras have been constructed, and if they 

 show indications of periodicity in its displays. Ka?nitz's" Meteoro- 

 logy," in which almost every feature of the weather capable of 

 being chronicled has been fully catalogued, probably contains a 

 list more or less complete, up to its author's time, of all then 

 known desciiptions of auroras. If this be so, it has probably 

 served for the groundwork upon which later and more complete 

 catalogues have been compiled, extended, and completed in his 

 own and other countries. Dr. Heis, the director of the 

 Prussian Observatory at Miinster, in Westphalia, is especially 

 active in collecting information of the slightest appearances of 

 aurora in any quarters of the globe, from whence published or 

 private descriptions of them can be obtained. livery succes- 



sive number of such works as Mr. G. J. Symons's Monthly 

 Meteorological Magazine and the Quarterly Journal of the 

 Scottish Meteorological Society contains, in a few pages of 

 "meteorological notes" on the weather peculiarities of each 

 month from their numerous observers, a list of scattered aurora- 

 observations, which is probably as complete for the British Isles 

 during the years in which these publications have been carried 

 on, as the perfect or partial clearness of the sky over this 

 country, and indeed over some adjacent continental stations, 

 enables such a list to be mide by obsarvations. But this collec- 

 tion, invaluable as it is for our own immediate field of registry, 

 is not assorted, nor suited, without extension by the help of 

 similar collections made in surrounding foreign countries, to be 

 regarded as a sufficiently extensive list of auroras for dealing gene- 

 rally with the question of their perioiicity. The present state of 

 progress of our knowledge, with regard to auroral frequency, we 

 owe largely, if not almost entirely, to the researches of Prof. E. 

 Loomis, of Yale College, U.S., the results of whose discussion 

 of the collateral views and considerations involved in them will 

 be found in numbers of the American fournal of .Science for 

 July 1S60, SejJt. 1S70, and April 1S73. In the first of these 

 papers, a map of lines of equal auroral frequency for the northern 

 hemisphere is presented, dividing the northern area of the globe 

 into zones encircling the arctic regions. It appears, for example, 

 from this map, that auroral displays are not very much more 

 frequently visible in St. Petersburg than they are in London, 

 and that even Boston and Edinburgh are as frequently visited by 

 them as the great northern capital itself. An oval belt of 



ih of the year (Kaemtz). 



greatest auroral frequency encloses together the north geographici 

 and north magnetic poles, covering all the European, Asiatic, and 

 American coast lines of the Arctic Ocean, and passing onwards 

 fiom the latter across Hudson's Bay, the mouth of Baffin's Bay, 

 and Iceland, back to the North Cape. For a short distance 

 within this ample belt auroras continue to be tolerably frequent, 

 and grow comjiaratively more scarce in Smith's Sound and the 

 northern parts of Baffin's Bay, and indeed apparently in propor- 

 tion as the geographical north polar regions are approached.* It 

 is with the outer and not with the inner margin, however, of this 

 ring-maximum of auroras that observers in ordinary latitudes are 

 concerned, and it is pointed out in his most recent paper by 

 Prof. Loomis, that in constructing general catalogues for de- 

 ciding questions of auroral periodicity, a line, or at least a re- 

 stricted zone, bordered northwards and southwards by lines of 

 equal auroral frequency, should be chosen as the localities from 

 which observations may be gathered. To jilace this line or belt 

 in the zone itself of almost constant auroral activity, where 

 auroras can only vary periodically in brightness rather than in 



* Jl will ba remembered that in Capt. Kane's descrip ion of a winter- 

 detention of his vessel in Smith's Sound (the northernmost p;issige from 

 Baffin's l?ay, about eight and-a-half degrees from the north pole), it is related 

 that the feeling of prolonged darkness at length became so oppressive that 

 even the Esquimaux dogs were aftected by it, and when excluded from the 

 luxury whined piteousjy for light. A darkness so deep and endurine as this 

 description suggests can scarcely have been broken, as it occurs in the more 

 favoured belt twenty degrees south of this high latitude, at the moutti of 

 Davis Strait, by the illumination of bright rays and flashing beams of con- 

 stantly appearmg line auroras. The position occupied by Capt. Kane was 

 not more than two or three degrees from the general centre of the region of 

 fast-diminishing auroral frequency, embracing the whole Aiciic Ocean, which 

 is shown on Prof Loomis's auroral map as merging insensibly on all sides 

 into the broad or narrow belt of greatest auroral activity surrounding it. The 

 latter seems to follow very nearly along its whole extent, with a correspond- 

 ing strong depression and expansion of its width towards Hudson's Bay, the 

 general direction of the arctic coast-line. 



