344 



NATURE 



[January 3, 19 18 



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 of Nature. No notice is 

 taken of anonymous communications.] 



Magnetic Storm and Aurora, December 16-17. 



The ioUovving details of a noteworthy magnetic 

 storm and aurora, which occurred on December 16-17, 

 are communicated by permission of the Director of the 

 Meteorological Office. C. Chkee. 



Kew Observatory, Richmond. 



On the magnetic traces at Kew Observatory, Rich- 

 mond, Surrey, there were indications of disturbance 

 shortly after 8h. on December 16, but no striking 

 movements until after i4h. (2 p.m.). Activity was 

 greatest between i6h. and midnight of December 16, 

 but some considerable movements appeared after mid- 

 night, and the disturbance did not die down until 

 after 4h. on December 17. The range of declination 

 (D) was about 34', the ranges of horizontal force (H) 

 and vertical force (V) being respectively about 400 7 and 

 2507. The needle reached its extreme westerly posi- 

 tion about i4h. 40m., and its extreme easterly position 

 just after 2ih. Its largest continuous movement was 

 a swing of 23' to the west, occupying about thirty-five 

 minutes, and ending just after 22h. The highest and 

 lowest values of H occurred about lyh. and 2ih. 15m. 

 respectively; between these hours there was a general 

 tendency to fall. A very rapid movement in H ended 

 just before 2ih. 15m., the element falling 2157 in 

 less than ten minutes. Between the end of this move- 

 ment and 2h. 25m. on December 17, H rose almost 

 350 y. 



From i4h. 30m. until after 2ih. on December 16 

 the D trace showed, superposed on a gradual drift to 

 the east, a series of oscillations with a mean period of 

 about twenty-two minutes. The H trace also showed 

 a series of oscillations between i5h. and i8h., and the 

 oscillations in the two elements were roughly in phase, 

 increase in H going with westerly movement of the 

 needle. The changes in V were of a normal kind, the 

 value of the element being raised between i5h. and 

 22h. on December 16, and depressed in the early hours 

 of December 17. The V trace was almost free from 

 short-period oscillations, and these were also less con- 

 spicuous in the D and H traces than is usual with so 

 large a disturbance. 



The following particulars are reported from Eskdale- 

 muir Observatory, Dumfriesshire, where the magneto- 

 graphs record the north (N), west (W), and vertical (V) 

 components of magnetic force : — 



Time of commencement 8h. 17m. G.M.T. on 

 December 16. 



h. m. 



Maximum of N at 17 23) 



Minimum ,, „ 21 27I 



Maximum of W ., 17 i6\ 



Minimum ., „ 21 141 



Maximum of V between 17 20'] 



and 17 35r>579y * 

 Minimum „ at 21 i8j 



These ranges, it will be noticed, especially that in 

 V, are much larger than those recorded at Kew Ob- 

 servatory. 



After the occurrence of the minimum values about 

 2ih. 20m., the Eskdalemuir curves showed a recovery 

 to about their normal positions ; but just before 2h. on 

 December 17 another disturbance was recorded, caus- 

 ing an increase in W and fall in N and V, the changes 

 NO. 2514, VOL. 100] 



of force being approximately in N — 150 7, in W + 80 y, 

 in V— 26Q.y. 



Observations of aurora on December 16 have been 

 reported from many stations in Scotland and Ireland. 

 At Eskdalemuir aurora was first noticed at 2oih., when 

 it appeared as a glow to W.N.W. At 2ih. an arc 

 extended from about N.W. to about N.E., with green 

 streamers extending towards the zenith. At 2ih. lom. 

 the arc had disappeared, but the whole northern half 

 of the sky up to the zenith was glowing brightly. At 

 2i5h. the glow was less extensive. It was noticed at 

 this time that the streamers radiated to a point about 

 10° from the zenith towards the south (i.e. the radiant 

 point had an elevation of about 80°, an azimuth of 

 about 180°). The streamers were not thin and sharply 

 defined, as is usually the case, but vaguely defined 

 patches of light which glowed brightly. At 2i^h. the 

 principal glow was to the W. and W.S.W., but it was 

 also plain to the N.W., N., and N.E. The natural 

 inference was that the centre of the arc of the horizon 

 from which the disturbance proceeded had changed 

 azimuth from N (nearly) to W (nearly) between 2i|h. 

 and 2i|h., but the radiant point of the streamers did 

 not change appreciably in position. A slight glow was 

 still visible in the N.E. at 23h. 



At Aberdeen Observatory, Mr. Clarke, the observer, 

 reported a fine auroral display on Decemlier 16. From 

 i6h. 45m. to 2ih. it was of a comparatively stable 

 character. Until i8h. there was a single curtain-arc, 

 with crimson, yellow, and green colouring. Between 

 i8h. and 2ih. there were several similar arcs, coloured 

 from yellowish-green to bluish- white. After 2 ih. the type 

 altered, streamers appearing all over the sky, accom- 

 panied by a corona. This second auroral phase would 

 seem to have synchronised with the very rapid fall of 

 magnetic horizontal force at Richmond. 



At Rothesay, according to the observer, Mr. J. 

 Davidson, the aurora was very fine indeed. Along 

 with extra long streamers were waves of red and white 

 light, the whole centring overhead and forming an 

 immense " dome " (corona), where both streamers and 

 waves of light centred. The red waves came mostly 

 from N.W. and N.E. 



At Fort Augustus the " dome " was in the zenith at 

 2ih. 15m. The most southerlv station from which 

 observations have been received at present is Seskin, 

 near Waterford, where the aurora was "moderately 

 bright" at 2ih., and' "faint" at 22h. The observer, 

 Mr. Ernest Grubb, writes: — "The aurora on Sunday 

 was much brighter at Mount Mellick, fiftylseven mile's 

 north of here, and very much brighter at Belfast, 

 jjd miles north of here." 



At Southport " a very fine display of streamers " was 

 seen early on December 17, between 2|h. and 2|h., 

 and therefore corresponded with the second magnetic 

 disturbance at Eskdalemuir. 



Range 

 6387 



5877 



SOURCES OF POTASH. 



AS is well known, the world's supply of potash 

 -'*■ during the last three years has been greatly 

 curtailed owing to the present isolation of Ger- 

 many, and compounds of potassium have, con- 

 sequently, greatly increased in price. This, 

 of course, has acted adversely on the interests of 

 agriculture, of medicine, and of numberless pro- 

 cesses in the arts which are more or less dependent 

 upon the use of potash compounds. Up to within 

 comparatively recent times such potash as the 

 world needed was obtained from sea-water, either 

 directly, or indirectly through the medium of sea- 



