464 EEPOET — 1881. 



Mr. Ellis, of the Greenwich Observatory, has shown the intimate re- 

 lation between solar action and the regular diurnal magnetic changes 

 of declination and horizontal force at Greenwich Observatory during 

 thirty-five years, from 1841 to 1876, by a comparison of the observations 

 of those elements. The results of his observations are recorded in a 

 valuable paper published in the ' Philosophical Transactions of the Royal 

 Society' (vol. 171, part ii. 1880), and they show what a close relation- 

 ship exists between solar activity and terrestrial magnetic changes. 

 There are not only daily and yearly periods of the variations of dif- 

 ferent magnetic elements, but there also seems to be in the horizontal 

 force a period of 25 or 26 days, which is the time of rotation of the 

 sun on his axis. Such a period would be a very strong argument in 

 favour of regarding the sun as a magnetic body, in which, as in the 

 earth, the axis of rotation does not coincide with the magnetic axis. 



Other recent investigations have shown that these regular magnetic 

 changes depend not only on the sun, but that they are also in part due 

 to the action of the moon ; and these portions depend upon the length of 

 the lunar day, and on the position of the moon with regard to the earth. 

 Just as there are regular earth-currents, whose direction depends upon the 

 sun, which we may call the solar earth-currents, so there are lunar earth- 

 currents which go through their changes under the action of the moon ; and 

 it has been shown that the effects are produced, not immediately under 

 the moon, but that there is a lagging behind in the case of the lunar earth- 

 currents, just as in the case of the solar earth-curi-ents. In the case of 

 the lunar earth-currents we cannot attribute the production of the elec- 

 tricity either to heat or to thermo-electric currents from one part to 

 another of the earth's crust, and we must therefore look for some other 

 source. May we not find it in the fact that the moon causes tides in the 

 solid crust of the earth, just as she causes tides in the oceans ? The earth's 

 crust is made up of elastic materials and materials capable of yielding and 

 altering their form to a considei-able amount with the change in the direc- 

 tion of the pull of the moon upon them. 



This crust also contains magnetic substances in abundance, which alter 

 their form vender the moon's attraction, and so, from the changes of rela- 

 tive position of masses of magnetic matter, changes are produced in the 

 magnetism of the earth, which must give rise to induced currents of 

 electricity or earth- currents. Let us imagine a conductor of electricity 

 outside the earth, stretching from the north pole to the equator, and 

 fixed in space, with the earth, a magnetic body, revolving beneath it from 

 W. to E., then it follows, from Faraday's laws of induced currents, that 

 the revolution of the earth on its axis would cause a current in the fixed 

 conductor in a direction from the pole to the equator. 



If the conductor moved over the surface of the earth from W. to E., 

 and the earth did not revolve, or revolved at a slower rate, then the 

 current in the conductor would be from the equator to the pole. The 

 current depends upon the relative motion of the earth and the wire. If, 

 then, we have an insulated wire running north and south, the tides in 

 the earth's crust, of which I have spoken, will be equivalent to a lagging 

 behind of magnetic matter, and so we may expect in that wire a current 

 of electricity whose general direction would be from the equator to the 

 pole. The position of the wire with reference to the magnetic pole of 

 the earth would modify the direction of these earth-currents, and it 

 is quite conceivable that the position of England with regard to the 



