i84 



NATURE 



[Jan. 3, 1878 



ever, been studied. The accompanying diagram will 

 show this for the last three minima. 



If we suppose that the mean diurnal movement of the 

 magnetic needle is determined for each month, we obtain 

 the amount of the oscillation or range ; the mean of the 

 ranges thus found for twelve successive months is repre- 

 sented by a point in the curves ; thus the last point in the 

 lowest curve represents the mean of the ranges for the 

 twelve months, October, 1876, to September, 1877 (corre- 

 sponding to April I, 1877), as shown by the observations 

 made in the Trevandrum Observatory (nearly on the 

 magnetic equator). The point immediately preceding 

 represents the mean range for the twelve months, 

 September, 1876, to August, 1877, and so on for the other 

 points. 



If these curves are examined, it will be seen, that in the 

 upper one the minimum is very clearly marked by two 

 points corresponding to April i and May i, 1856 (repre- 



senting the mean ranges, October, 1855, to September, 

 1856, and November, 1855, to October, 1856), and that 

 there is little difference in the rapidity with which the 

 curve descends to, and ascends from, the minimum. 



In the middle curve the epoch of minimum is by no 

 means so distinctly marked ; it occurs between the points 

 for April i and September i, 1866. There is also a con- 

 siderable difference in the rapidity of variation in the 

 descending and ascending branches of the curve. The 

 descent is nearly as rapid as in the upper curve, but the 

 ascent is very much slower. 



In the lower curve, the lowest point is that for Decem- 

 ber I, 1875, but it is even now, with points for a year and 

 a half later, difficult to say whether this is the minimum 

 or not, the point for January i, 1877, being only o'"02 (two- 

 hundredths of a minute of arc) higher. In this curve 

 the change of range in the diurnal oscillation is quite 

 insignificant from November i, 1874, to April i, 1877, 

 including the ranges from May i, 1874, to September 30, 

 1877, an interval of three years and five months. If this 

 result is confirmed by other observations, as I believe 

 will be the case, no such constant state of the sun's mag- 

 netic action will have been observed since the last years 

 of the eighteenth century. 



The observations of sun-spots, even if they give as 

 accurate a measure of the intensity of the cause as that 

 obtained from the movements of our magnets, cannot be 

 observed with the same continuity, nor be measured with 

 the same precision ; but I have little doubt they will con- 

 firm generally the result shown in the last curve, as they 

 have in preceding casc3. 



With regard to the aurora borealis, the appearances 

 seem to have been'very rare during the last two winters. In 

 the report by Capt. Sir G. Nares, on the Arctic expedition, 

 he says that in the winter of 1875-76, " Light flashes of 

 aurora were occasionally seen on various bearings,but most 

 frequently passing through the zenith ; none were of suffi- 

 cient brilliancy to call for notice. The phenomena may be 

 said to have been insignificant in the extreme, and, as far 

 as we could discover, were totally unconnected with any 

 magnetic or electric disturbance " (Nature, vol. xv. 



P- 35)- 



In the twelve months including September, 1843, and 

 August, 1844, including the epoch of minimum disturb- 

 ance and of auroral frequency, I observed in the south 

 of Scotland (in lat. 55" 35') thirty appearances of the 

 aurora, and from September, 1844, till the end of 

 1845, fifty-nine appearances were observed at this single 

 station.^ Making every allowance for the continuous 

 watch over the magnetic instruments at the Makerstoun 

 Observatory during these years, the difference between 

 Capt. Sir G. Nares' result, in so high latitude, in 1875-76, 

 and that for the south of Scotland, is very distinct. I 

 ought to add, with reference to the apparent want of 

 connection of the faint auroral appearances with the 

 magnetic disturbance noticed by Sir G. Nares, that seve- 

 ral of the aurora; observed by me were of the very 

 faintest kind, mere " traces," as I have termed them, 

 which I could never have remarked had I not been 

 warned by very slight magnetic irregularities to examine 

 the sky with the greatest attention. Again, in no case 

 have I seen the faintest trace of an aurora without finding 

 at the same time a corresponding irregularity in the 

 movement of the force or declination magnet. 



I am unacquainted with any observations of the aurora 

 made in the British Isles during the last two winters ; * I 

 believe that no scientific institution exists in this country 

 which makes the look-out for aurora throughout the night 

 a definite portion of its-workj and that all our knowledge 

 of this phenomenon appears to be left to tlie^ chances of 

 some one being out, at the hour of a display, sufficiently 

 bright to attract his attention who will take the trouble to 

 communicate his observation jo a public journal. 



John Allan Broun 



P.S. — I have to thank Mr. A. Buchan for kindly fur- 

 nishing me with a note of the auroras seen at the stations 

 of the Scottish Meteorological Society during the ye.7.r 

 1876. These amounted to forty-two in number, twenty- 

 six in the first half and sixteen in the second half of the 

 year. The greater part were seen in the most northerly 

 stations, including the Orkney, Shetland, and Faro 

 Islands ; nine only having been seen south'^of the Forth. 

 I cannot, however, compare the total result from the 

 hundred stations of the Society with that from the single 

 southerly station of Makerstoun in 1844, since much 

 depends on the nature of the watch kept in each case. It 

 is, however, gratifying to find that so much attention is 

 given at the stations of that highly useful scientific body, 

 the Scottish Meteorological Society, to the observation of 

 this phenomenon. 



December 31, 1877 



1 "General Results of the Makerstoun Observations," p. Ixxv., Trans. 

 Roy. Soc. Edin., Part 2, vol. xix. 



2 I do not omit Mr. Kinahan's account of "auroric lights," which he saw 

 so frequently in the winter of 1876 77, and which he considered a species of 

 aurora borealis (Nature, vol. xv. p. 334), as I think there must have been 

 some mistake as to the nature of those lights. He says they were " very 

 common and brilliant during 'the dark days' of December, a few hours 

 before dawn (about five o'clock)." The aurora borealis is very rarely seen 

 at five A.M in this country. In the two years, 1844 and 1845, during which 

 the aurora was sought for at Makerstouu every hour of the night, it was 

 observed on seventy-seven nights on an average of nearly three hours each 

 night, but it was seen only twice so early, and that with a bright or brilliant 

 aurora which remained during five hours on the first occasion, and from six 

 P.M. to ^ix A.M on the second. I cannot say, also, that I have ever seen 

 paftlof the phenomenon desctibed by Mr. Kinahan, and I had hoped that 

 some other observer in Ireland would have confirmed his observations, 

 which if exact, would be most important, especially as made so frequently at 

 the epnch of ra'nimuii. 



