VAKIATIONS OF THE MAGNETIC NEEDLE. 281 



equalized, by multiplying the former by constant coefficients. The 

 curves of both lines exhibit a general resemblance to the course of 

 the diurnal variation of the declination ; but the influence of the 

 horizontal force is also very evident, especially in the afternoon 

 branch of the curve, where its effect is to retard the return from 

 the minimum at 1 P. M. This effect is much greater in the Derby 

 and Birmingham line than in that of Derby and Eugby, the 

 azimuth of the former line, measured from the magnetic meridian, 

 being much greater than that of the latter. The agreement of 

 the calculated with the observed curves is probably as close as 

 could be expected in the results of so short a series ; and we seem 

 entitled to conclude that the diurnal movements of the two mag- 

 netometers are justly accounted for by electric currents traversing 

 the upper strata of the earth. 



10. Upon a closer examination of the two sets of curves, how- 

 ever, there are found some differences to which it is necessary to 

 advert. In the first place, the turning points of the calculated 

 curves are generally later than those of the observed, by about one 

 hour. Thus, in the Derby and Birmingham line, the maximum 

 of the observed force occurs about 8 h O m A.M., and the minimum 

 about 12 h 40 m r. M. The corresponding times for the calculated 

 force are 9 h O m A. M., and l h 40 m P. M., nearly. We shall presently 

 find grounds for believing that time may possibly be required, in 

 order that the current may produce its full magnetic effect. 



11. Another discrepancy is, that the calculated curve is, for 

 the most part, above the observed, especially in the Derby and 

 Birmingham line. This will be evident if the two curves be 

 referred to the same axis of abscissae. It is probably to be accounted 

 for by the fact, that the zero from which the magnetic deflections 

 are measured is not the true one, corresponding to the absence 

 of deflecting force. As we have no means of determining the 

 latter, we are accustomed to take the mean position for the entire 

 day, or the mean of the readings taken at equal intervals, as the 

 point from which the deflections are measured. But there is 

 reason to believe that this is not the true position of rest, corre- 

 sponding to the absence of all disturbing force. The comparative 

 quiescence of the magnets, during the early hours of the morning, 

 seems to indicate that they are then near their true positions of 

 equilibrium ; and this indication is confirmed by the galvanometric 

 curves, the zero-line, which corresponds to the absence of all current, 



