660 Professor W. G. Adams [June 3, 



which may be due to an electric wave extending over a considerable 

 area of the earth's surface. The point towards which the total earth 

 current is directed follows the sun and seems to lag two or three 

 hours behind, but not the same distance behind at diiferent places. 



These earth currents have been ascribed to different causes : thus 

 Dr. Lamont regards them as the results of electric force emanating 

 from the sun ; De Saussure regards them as developed by evaporation, 

 the vapour being positively charged and the water being negative ; 

 Dr. Lloyd regards them as effects of solar heat ; whilst M. de la Eive 

 ascribes them to chemical actions going on in the interior of the solid 

 crust- of the earth, the electricity being transported into the atmo- 

 sphere by evaporation. Mr. Ellis, of the Greenwich Observatory, has 

 shown the intimate relation 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 shown on a large diagram which has been 

 enlarged from his curves (published in the Phil. Trans, part ii. 1880, 

 p. 541), and they show what a close relationship exists between solar 

 activity and terrestrial magnetic changes. There are not only daily 

 and yearly periods of the variations of the different magnetic elements, 

 but there also seems to be in the horizontal intensity a period of 

 twenty-five or twenty-six days, which is the time of rotation of the 

 sun on his axis. 



Other recent investigations have shown that these regular mag- 

 netic changes dej)end 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 there is a 

 lagging behind in the case of the lunar earth-currents just as in the 

 case of solar earth-currents. In the case of the lunar earth-currents 

 we cannot attribute the production of the electricity 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 considerable amount with the change in the 

 direction of the pull of the moon upon them. This crust also contains 

 magnetic substances in abundance, which alter their form under the 

 moon's attraction, and so from the changes of 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 



