314 Royal Institution. 



reached by voyagers ; and two (the Siberian pole and the Cape Horn 

 pole) being only inferred from the convergence of the directions of 

 the needle. It had, however, been shown by M.Gauss that a theo- 

 retical investigation which would give as one of its consequences this 

 convergence of directions would also negative the existence of the 

 supposed poles ; and on the whole the lecturer expressed himself 

 as now doubtful of the existence of those poles. He expressed his 

 regret that an idea long ago explained by Prof. Christie had not been 

 followed out, namely, of the preparation of charts showing the lines 

 perpendicular to the direction of the needle. A chart of the entire 

 magnetic force was exhibited ; and the general fact of no dip near 

 the equator, and increasing dips of the austral end of the needle in 

 the north, and the boreal end in the south, was again mentioned. 



The next class of facts mentioned was the diurnal variation : — 

 that in northern latitudes the austral end of the horizontal needle 

 points furthest to the east at about eight in the morning, and furthest 

 to the west at about two in the afternoon. In southern latitudes 

 the change is in the opposite direction ; and in low latitudes, as at 

 St. Helena (Col. Sabine), and on the Red Sea (M. d'Abbadie) the 

 change has the north-latitude character during the north-latitude 

 summer, and the opposite character during the opposite season. 

 The horizontal and vertical forces generally increase from morning 

 to evening. 



The third class of facts was the momentary changes first brought 

 to light by the observations of the German Magnetische Ferein 

 (above-mentioned). These had appeared to the lecturer so import- 

 ant, that, principally for the better recording of them, he had brought 

 before the British Association the importance of recommending to 

 the Government to hold out the prospect of pecuniary reward to the 

 inventors of effective self-registering apparatus. The Government 

 had liberally responded to the application, and the consequence was, 

 that most beautiful and effective photographic self-registering appa- 

 ratus, constructed respectively by Mr. Brooke and Mr. Ronalds, 

 had been combined with the free magnet, the bifilar magnet, and the 

 vertical-force magnet, and had now been brought into daily use at 

 the Royal Observatory of Greenwich, and at Toronto ; and that 

 their use appeared likely to extend. The general fact exhibited by 

 the five-minute observations and by the photograph record is this : — 

 The changes of direction of the horizontal needle and of the hori- 

 zontal and vertical forces are incessant, and as examined at any one 

 place appear most capricious. But if compared at several neigh- 

 bouring places, for instance not exceeding 500 miles apart, they 

 are found to be exactly similar : if the distance be increased, the 

 similarity diminishes ; and if places be selected spread all over the 

 globe, it is usually found that a large disturbance at one place is 

 accompanied by large disturbances at the other places, in which 

 however it is difficult to trace the relation of the contemporaneous 

 movements at the different places. Diagrams exhibiting these phse- 

 nomena were placed before the meeting. The lecturer pointed out 



