?o6 



NATURE 



[January 5, 191 r 



the subject. When my book was originally planned it 

 was intended that it should be a monograph of the speci- 

 mens of okapi contained in the national collection, and 

 it thus became entered on our list as " the monograph 

 on okapi." 



More, no doubt, might be written about the specimens 

 which I had under examination, and I should have, in 

 some circumstances, been able to add to what the book 

 contains ; but the problems which arose in the course of 

 my work could not, in many cases, be satisfactorily solved 

 by the examination of the existing material. 



We shall have to wait for new observations made upon 

 fresh or living specimens for a solution of the question 

 as to what are the characteristics of the male and female 

 okapi respectively, what are their geographical variations, 

 and whether there are distinct races or subspecies. 



E. Ray Lankester. 



29 Thurloe Place, South Kensington. 



Sir E. Ray Lankester is correct in supposing that I 

 was misled by the last paragraph of the preface to his 

 work on the okapi into the belief that there had been 

 or might be an additional volume of text to supplement 

 the illustrations given in the volume under review. From 

 private correspondence which passed between Sir E. Ray 

 Lankester and myself about three years ago I was under 

 the impression that the " text " alluded to was in exist- 

 ence, and perhaps I arrived too hastily at the conclusion 

 that for reasons of economy it had been put aside because 

 of the intervening publication of M. Jules Fraipont's 

 work. The title " Monograph of the Okapi " to which 

 Sir E. Ray Lankester refers as likely to mislead an 

 appraiser of his work was not of my bestowal, but is the 

 official title of this valuable and admirably produced 

 volume. The illustrations are fully described ; but I 

 suppose what I missed, and what I hoped might still be 

 forthcoming, were the deductions to be drawn from these 

 illustrations as to the affinities and systematic position of 

 Okapia : in short, a statement of Sir E. Ray Lankester 's 

 personal opinions. He is probably quite right to with- 

 hold these until something is known of the beast's 

 musculature and intestines. 



H. H. Johnston. 



The Dynamics of a Golf Ball. 



With a view to reproduction in the forthcoming Life 

 of the late Prof. Tait, I have just been editing his popular 

 article on long driving, which appeared in the Badminton 

 Magazine of March, 1896. On reading Sir J. J. Thom- 

 son's lecture, as published in Nature of December 22, 

 19 10, I was greatly struck with the strong resemblance 

 between golf-ball paths worked out mathematically by 

 Tait and the stream lines of the electrified particles in the 

 ingenious experiment devised by Sir J. J. Thomson. A 

 few of Tait's calculated curves were given in Nature, 

 vol. xlviii. (June 29, 1893) ; but better examples will be 

 found in the second paper on the path of a rotating 

 spherical projectile (Trans. R.S.E., vol. xxxix., or 

 Scientific Papers, vol. ii., p. 386) and in the article on 

 long driving already mentioned. 



By laborious arithmetical calculations, Tait and his 

 assistant computer worked out a series of possible trajec- 

 tories with various values for the transverse force due to 

 the underspin, obtaining, among others, the kinked path 

 which Tait had already demonstrated by undercutting a 

 light rubber balloon. It is extremely interesting to see 

 how the several types of curve figured by Tait for the 

 same initial speed of projection, but varying degrees of 

 underspin, are almost accurately reproduced by Sir J. J. 

 Thomson's beautiful method of subjecting a stream of 

 negatively charged particles to a suitable combination of 

 electric and magnetic forces. C. G. Knott. 



Edinburgh University, January 2. 



On the Simultaneity of Abruptly-beginning 

 Magnetic Storms. 



I was naturally much interested in Dr. Krogness's com- 

 munication to Nature of December 8, 1910 (p. 170), and 

 wish to take this occasion to express my gratefulness to 



NO. 2149, VOL. 85] 



him for makmg known his criticisms on some of th-.- 

 results of my investigations on magnetic stornis, as well 

 as on those of Mr. Faris, where there is opportunity for 

 reply. I am also glad that he has made his statements 

 sufficiently direct, so as to admit of an equally direct 

 answer. 



Dr. Krogness first wishes to show that my conclusion, 

 that even the sudden magnetic disturbances do not begin 

 strictly at the same instant, but at measurably different 

 times at various points on the earth, rests on insecun- 

 foundation ; he would make it appear that it was based 

 on but two cases, viz. the disturbance of May 8. 1902 » 

 and that of January 26, 1903. He will find a table (No. 

 Vin.) in No. 2 of my researches (December, iqio, issu>; 

 of Terrestrial Magnetism and Atmospheric Electricity) 

 which summarises the data from thirty-eight abruptly- 

 beginning disturbances between the years 1882 and 1909, 

 thirty-four of which were available to me when the articl'- 

 was prepared which Dr. Krogness reviews (loc. cit., pp. 

 19-20). 



The table gives the date and approximate Greenwich 

 mean civil time for each of these thirty-eight disturbances, 

 next the number of observatories for which time data wer" 

 available and the approximate portion of a complete 

 circuit of the earth embraced by the contributing observa- 

 tories. Then the value of x, or the time in minutes re- 

 quired by a disturbance to pass over one-fourth of a great 

 circle, and in the following columns is given the approxi- 

 mate weight to be attached to any particular value of ;v» 

 as determined from all circumstances involved, and the 

 source from which the data have been obtained. A plus 

 sign attached to x means that the disturbance progressed 

 apparently in an eastwardly direction, as indicated by an 

 increase in the Greenwich mean time of beginning at 

 easterly stations over that at westerly ones. A minus 

 value of X means, of course, the reverse. Nos. 35-3S 

 were since added on the basis of data communicated by 

 Mr. Faris (loc. cit., pp. 213, 214). 



Out of thirty-eight values of x, only ten, or about one- 

 fourth, have the negative sign, so that three-fourths of the 

 disturbances of the type here considered show an eastwarcf 

 progression at the times of beginning. In view of the 

 greatly varying circumstances on which the figures are 

 based — different observatories, diffe/ent instruments, times 

 scaled by different persons, different years, covering a 

 period of two and a half times that of a sun-spot cycle — 

 it is going to be difficult to explain the persistency of the 

 plus sign by any such possible errors as Dr. Krogness 

 points out, which, as a matter of fact, even he will hardly 

 contend would be always in the same directi/Mi for every 

 observatory, nor even necessarily always the same at the 

 same station. 



From this table the following results are derived : — 



Weighted mean value of 28 plus values of x = + i "65 minutes 



,, ,, 10 negative ,, =-l"8o ,, 



Weighted mean without regard to sign ... = ±.i'6g „ 

 (Hence velocity of progression for average 

 sudden disturbance, whether to the east 



or to the west, is « ... 99 km. /sec.) 



Weighted mean with regard to sign ...- ... = -rO'74 minute 

 (Hence average algebraic velocity of east- 

 wardly progression is 225 km. /sec.) 



We thus get a velocity for the progression of a sudden 

 disturbance on the order of 100 to 200 kilometres per 

 second ; hence, if a sudden disturbance passed around the 

 earth completely it would take approximately between 

 seven and three minutes. We are here, then, dealing 

 apparently with a velocity of a greatly subordinate order 

 (1/3000 to 1/1500) to that of electromagnetic waves, which 

 would require but a tenth of a second to pass round the 

 earth, and of kathode rays which would take on the order 

 of a half-second. 



Another line of argument set forth in my papers is based 

 on the harmonic analysis of the typical disturbance here 

 under consideration, for which the effect, in general, is an 

 increase in H (horizontal intensity) over the whole earth 

 and a decrease in Z (vertical intensity) in the northern 

 magnetic hemisphere and an increase in Z in the southern. 

 It was found, for example, that the disturbance system of 



