VARIATIONS OF THE MAGNETIC NEEDLE. 287 



7 h 35 m P. M., it is west. Finally, after midnight it reaches its 

 stationary position to the west of north. 



The order of the changes is similar in summer and in winter. 

 The principal differences lie in the times of reaching the azimuths 

 and 270, the direction of the current being in the former 

 azimuth about 1^ hours earlier in summer than in winter, and in 

 the latter about two hours later. The time of the afternoon meri- 

 dian passage is nearly the same at the two seasons. 



The intensity of the current is greatest in the south and east. 

 The maximum intensity occurs at 1 P.M., the azimuth of the current 

 being then S. 35 E. There is a secondary maximum about 7 P.M., 

 preceded by p, secondary minimum about 5 P.M. 



A curious inversion, or fold of the curve, takes place during the 

 night, between 10 P.M. and 6 A.M. The nocturnal minimum occurs 

 between 1 A.M. and 2 A.M. ; and the direction of the current is then 

 nearly opposite to that of the maximum at 1 p. M. 



17. In the foregoing deductions, it is to be borne in mind, in 

 conformity with the remarks of Art. 13, that the current referred 

 to is not that actually subjacent to the place of observation, but the 

 resultant, for that place, of all the currents occupying a consider- 

 able portion of the earth's surface. The reasoning, in fact, relates 

 to the magnetic effects of the currents, rather than to the currents 

 themselves, and the immediate subject of calculation is the intensity 

 of the magnetic disturbing force, and the normal to its direction. 



It is further to be remembered, that the conclusions are affected 

 by the uncertainty to which we have before adverted (Art. 11), re- 

 specting the zero, or origin, from which the magnetic deflections 

 are to be measured. It is probable, however, that this uncer- 

 tainty does not materially affect the results, except at those hours 

 at which the magnetic variations are small, and when, of course, 

 any given change in the amount of the deflections bears a much 

 larger proportion to the whole. 



18. Although the laws of the diurnal changes of the magnetic 

 elements at different points of the globe have much in common, 

 they present nevertheless marked differences. Corresponding dif- 

 ferences must, therefore, be expected in the diurnal changes of the 

 earth-currents at different places ; and, consequently, the general 

 laws to which they are subject can only be known by a comparison 

 of the results at many places widely distributed over the globe. I 

 have accordingly thought that it would reward the labour to make 



