ON MAGNETIC DISTURBANCES AND EARTH-CDREENTS. 473 



storm -whicli took place on January 31 last, the aurora was well seen in 

 England ; it was also seen at St. Petei'sburg and as far east as Siberia. 

 It does not appear to have been seen, although it was looked for, at Zi- 

 ka-Wei in China by M. Dechevrens, the director of the Observatory, 

 although the magnetic storm was so violent there that the horizontal 

 force was suddenly changed by -roiyth part of its total amount. 



In his address before the British Association in 1863 Sir William 

 Armstrong speaks of the sympathy between forces operating in the sun 

 and magnetic forces on the earth, and notices a remarkable phenomenon 

 seen by independent observers on September 1, 1859. ' A sudden out- 

 burst of light, far exceeding the brightness of the sun's surface, was 

 seen to take place, and sweep like a drifting cloud over a portion of the 

 solar surface. This was attended with magnetic disturbances of unusual 

 intensity, and with exhibitions of aurora of extraordinary' brilliancy. The 

 identical instant at which the effusion of light was observed was recorded 

 by an abrupt and strongly-marked deflection in the self-registering 

 instruments at Kew. The magnetic storm commenced before and con- 

 tinued after the event.' 



The daily and yearly periods of the magnetic changes, the change 

 in the horizontal force depending on the sun's rotation on his axis, the 

 agreement of the eleven- year period of magnetic disturbances, sun-spots, 

 and auroras, show that the sun plays a very important part in causing or 

 regulating both the regular and irregular magnetic changes. 



"We know so little as yet of the causes of the changes of the sun, and 

 their connection with terrestrial phenomena, that we can hardly do more 

 than ask what possible causes there are which could account for the 

 efiecta which are observed. Can we suppose that the Sun is a very power- 

 ful magnet, and that a great alteration in his magnetism accompanies 

 the production of the bright faculee and the spots in his atmosphere ? 

 Such a change of magnetism would affect the magnetism of the earth, 

 although the effect could not be very large, unless the sun is magnetized 

 to an intensity much greater, even compared to his mass, than the earth 

 is magnetized. Then, as there are tides in the seas around us, and pro- 

 bably in the earth's crust, so there are certainly very large tides in the 

 ocean of air above us ; and may not the sun and moon, by dragging this 

 air towards them as the earth revolves, cause that friction between air and 

 earth, and also that evaporation, which, together, may account for the 

 presence of, and keep up the supply of, positive electricity in the air and 

 negative electricity in the earth ? Again, these tides in the atmosphere 

 will cause the mass of it to lag behind the revolving solid earth, and at a 

 height of thirty or forty miles we have a layer of air which, for air, is a 

 comparatively good conductor of electricity. Here, then, we have not a 

 lagging of the magnet behind the conductor, but a lagging of the con- 

 ductor behind the magnet, and hence we may expect a current or a gradual 

 heaping-up of electricity in the air in the opposite direction to the current 

 in the earth's crust. Thus, whilst the tidal wave in the earth's crust would 

 cause a current in a telegraph wire from the equator towards the poles, 

 the regular tidal waves in the atmosphere would cause the gradual 

 transfer of positive electricity in the atmosphere from the poles to- 

 wards the equator. This transfer may be of the nature of a current of 

 electricity or of a mass of air carrying a static charge of electricity with 

 it ; for, as Professor Rowland has shown that the motion of a static charge 

 will produce magnetism, so we may expect, from the principles of conser- 



