550 REPORT — 1894. 



1885 and 1889 it lost, so that the gain was rather more than compensated. The 

 difference of the declination of the two places appears to increase and diminish 

 through a range of five minutes of arc. 



This evidence can be supplemented by other equally significant examples. No 

 fact connected with terrestrial mao^netisni is more certaiu than that at present 

 the rate of secular change of declination in this part of Europe increases as 

 we go north. This is shown hj a comparison of our survey with those of our pre- 

 decessors fifty and thirty years ag(j, by M. Moureaux's results in France, and by 

 Captain Creak's collation of previous observations. Yet, in spite of this, Stonyhurst, 

 which is some i^OO miles north of C4reenwich and Kew, and should therefore outrun 

 them, sometimes lags behind and then makes up for lost time by prodigious bounds. 

 Between 1882 and 1886 the total secular change of declination at Stonyhurst was 

 about 3'o less than that at Greenwich and Kew, whereas in the two years 1890-92 

 it reached at Stonyhurst the enormous amount of 28', just doubling the correspond- 

 ing alteration registered in the same time at Kew. If these fluctuations are caused 

 by the instruments or methods of reduction, my argument in favour of frequent 

 comparisons and uniform treatment would be much strengthened ; but, apart from 

 the inherent improbability of such large differences being due to the methods of ob- 

 servation, the probability of their physical reality is increased by the work of the 

 magnetic survey. 



The large number of observations at our disposal has enabled us to calculate the 

 secular change in a new way, by taking the means of observations made about five 

 years apart at numerous though not identical stations scattered over districts about 

 150 miles square. The result thus obtained should be free from mere local varia- 

 tions, but as calculated for the south-east of England for the five j'ears 1866-91 it 

 differs by nearly 5' from the change actually observed at Kew. 



We have also determined the secular change at twenty-five stations by double 

 sets of observations made as nearly as possible on the same spot at intervals of 

 several years. The results must be interpreted with caution. In districts such as 

 Scotland, where strong local disturbances are frequent, a change of a few yards in 

 the position of the observer might introduce errors far larger than the fluctuations 

 of secular change. But when all such cases are eliminated, when all allowance is 

 made for the possible inaccuracy of field observations, there are outstanding varia- 

 tions which can hardly be due to anything but a real difference in the rate of 

 change of the magnetic elements. 



A single example will suflice. St. Leonards and Tunbridge Wells are about 

 thirty miles apart. Both are situated on the Hastings Sand formation, and on good 

 non-magnetic observing ground. At them, as at the stations immediately around 

 them — Lewes, Eastbourne, Appledore, Etchingham, Ileathfield, and Maidstone — 

 the local disturbing forces are very small. All these places lie within a district 

 about forty miles square, at no point of which has the magnet been found to deviate 

 by 5' from the true magnetic meridian. No region could be more favourably 

 situated for the determination of the secular change, yet according to our observa- 

 tions the alteration in the declination at St. Leonards in .six years was practi- 

 cally equal to that at Tunbridge Wells in five. It is difficult to assign so great a 

 variation to an accumulation of errors, and this is only one amongst several 

 instances of the same kind which might be quoted. 



AVe find, then, when we consider the earth as a whole, grave reason to question 

 the old idea of a secular change caused by a magnetic pole or focus pursuing an 

 orderly orbit around the geographical axis of the earth, or oscillating in some 

 regular period in its neighbourhood. It would, of course, be absurd to admit the 

 possibility of change in the tropics and to deny that possibility in the arctic 

 circle, but the new facts lead us to look upon the earth not as magnetically inert, 

 but as itself — at the equator as well as at the pole — producing or profoundly modify- 

 ing the influences which give rise to secular change. And then, when we push our 

 inquiry further, accumulating experience tells the same tale. The earth seems as it 

 were alive with magnetic forces, be they due to electric currents or to variations in 

 the state of magnetised matter. We need not now consider the sudden jerks which 

 disturb the diurnal sweep of the magnet, which are simultaneous at places far apart, 



