16 SECTIONAL ADDRESSES. 



a current in the inner coil system it was easy to produce a symmetrical 

 magnetic field about tlie outer sphere, but no true representation of the 

 actual magnetic state of the earth could be obtained with currents in the 

 two coil systems alone, and Dr. Wilde, after reasoning that the underwater 

 portions of the earth were the more susceptible to magnetisation, covered 

 with sheet iron those portions of the interior surface of the outer globe 

 which corresponded to the oceans. Small sheets of iron were also placed 

 on areas occupied by great mountain ranges. The results of experiments 

 with this model were certainly interesting, for the general shape of the 

 magnetic field of the earth was remarkably well reproduced. The main 

 idea was that electric currents circulated in the inner regions of the earth 

 which were supposed to be of higher conductivity than the outer crust, 

 but in this latter currents of lower intensity also circulated, but their 

 distribution was subject to considerable variations by the shielding effect 

 of the ocean areas and mountain ranges. It is of interest to note that 

 the modern view of the earth's conductivity is that of an outer shell 250 

 km. thick of comparatively low conductivity and an inner sphere to 

 which a conductivity of about 3"6 X 10'^ C.G.S. units is attributed, the 

 magnetic permeability being unity. 



Terrestrial magnetic science has a long history, but much of the data 

 on which theories have been based are not very exact. The first concep- 

 tion of the earth as a great magnet appears to be due to Gilbert, but it 

 was not until Gauss made his analysis in 1838 that the nature of the earth's 

 magnetism and its distribution were made clear. Shortly after the work 

 of Gauss international co-operation in magnetic work was initiated by 

 Humboldt and Gauss and supported by Herschel, Kupffer and Sabine. 

 Six observatories were established in Russia under the direction of Kupffer 

 and three were established in the United States, another at Simla and one 

 at Singapore. Originally it was proposed to carry on the work of these 

 observatories for three years, but owing to delays the period was extended 

 to six years, and so desirable did it appear to continue this international 

 work that a Magnetic Congress was called and held at the Cambridge 

 meeting of this Association in 1845. 



The principal question which that conference had to decide was 

 whether the combined system of British and foreign co-operation for the 

 investigation of magnetic and meteorological phenomena, which had then 

 been five years in progress, must be broken up. 



It is scarcely necessary to say that the Congress was not divided on 

 this question, and resolved 



' that the cordial co-operation which has hitherto prevailed between 

 the British and foreign magnetic and meteorological observatories, 

 having produced most important results and being considered by us 

 as absolutely essential to the success of the great system of combined 

 observation which has been undertaken, it is earnestly recommended 

 that the same spirit of co-operation should continue to prevail.' 



The spirit of co-operation which existed then exists still, but I venture 

 to ask ' Do we make our plans suflB.ciently well 1 ' 



When data obtained in a small laboratory cannot suffice for the 

 elucidation of a problem, and when a chain of laboratories, either in one 



