35 6* Mr. A. Schuster on the Diurnal Period 



minimum of vertical force at 11 a.m. ; but there is no pro- 

 nounced maximum ; two minor maxima occur, one at 6 a.m. 

 and the other at midnight. 



As far as these results go, they give an emphatic answer in 

 favour of the supposition that a great part, at any rate, of the 

 disturbing currents lie outside the earth's surface ; a view 

 which Prof. Balfour Stewart has often supported in the last 

 few years. 



The results seem to me very encouraging, and I hope soon 

 to be able to make use of more material, and to obtain more 

 accurate expressions for the various forces concerned. 



It would help considerably all those who, like myself, wish 

 to obtain some knowledge on the subjects of terrestrial mag- 

 netism without the aid of a staff of computers, if the directors 

 of magnetical observatories were to reduce their observations 

 so as to give us changes in the components of force directed 

 towards the geographical north and west, rather than, as is 

 customary, changes in horizontal force and declination. In 

 all reductions such as I have attempted, and indeed in all 

 comparisons of the results obtained at different stations, what 

 interests us most is the two components of force resolved 

 along two definite directions like north and west, and not the 

 components of force resolved, as at present, in directions 

 changing sometimes rapidly from place to place. 



It may ultimately appear that some variations of terrestrial 

 magnetism are expressed most simply according to a system 

 of large and small circles, having the points of intersection of 

 the magnetic axis with the surface of the earth as poles ; but 

 the present system seems to me quite arbitrary, and until we 

 know more accurately the position of the magnetic axis, the 

 latitude and longitude circles seem to me to be the only 

 possible lines of reference for the magnetic forces. 



Supposing we have, by expansion in spherical harmonics, 

 obtained the distribution of potential, representing some peri- 

 odic variation we wish to investigate, we may easily, should 

 it be considered desirable, obtain such a distribution of elec- 

 tric currents on any sphere concentric with the earth's sur- 

 face, inside or outside as the case may be, as might cause the 

 observed variation. Such a representation would always be 

 instructive, although the actual currents causing the disturb- 

 ance may be distributed in a very different manner if they 

 have any vertical components. The simplest plan would be, 

 in the first place, to take the sphere on which we draw the 

 currents close to the earth's surface, either just outside or just 

 inside, according to the result obtained for the vertical force. 



If the magnetic potential is distributed over the earth's 



