AND THEIR CONNECTION WITH MAGNETIC CHxlNGES. 369 



range of the latter, and that this does not depend on the absohite amount of the 

 disturbing force, magnetically measured. 



A single glance at the Kew disturbance curves will show any one adopting 

 the induction hypothesis why the range of the earth-currents observed during 

 disturbances is so very great. The chief characteristic of a disturbing force will 

 then be seen to consist rather in ahriiptness than in absolute amount of change ; 

 and it may safely be affirmed that the abruptness of a disturbance exceeds that 

 of the ordinary daily variation to a very much greater extent than the absolute 

 amount of deviation from the normal caused by a disturbance exceeds the ordi- 

 nary daily range. 



But, on the induction hypothesis, the values of the earth-currents observed 

 during disturbances will depend upon the abruptness of the latter — change in one 

 direction producing positive, and that in the opposite direction negative currents ; 

 hence the range of such currents will be very great. 



Dr Lloyd again remarks, — " The calculated curve is for the most part abate 



the observed, especially in the Derby and Birmingham line It 



may probably 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 accus- 

 tomed 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, corres- 

 ponding 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 

 current, dividing the area of the diurnal curve unequally, and being nearer to the 

 night observations than to those of the day." 



The following is the explanation of this fact afforded by the induction hypo- 

 thesis : — " During the early hours of the morning, when the magnets are com- 

 paratively quiescent, and there is hardly any magnetic change, there will <!on- 

 sequently be hardly any induced current, and hence the observations during these 

 hours will approach very near to the zero of current. 



The results of this investigation may be briefly stated as follows : — 



1st, The earth-currents observed during periods of magnetic calm follow a 

 well-marked daily law, one feature of which is the small value of those currents 

 collected during the early morning hours ; and this admits of being readily ex- 

 , plained on the induction hypothesis. 



2d, These observations are probably influenced by such meteorological con- 

 ditions as affect the electrical conductibility of the upper strata of the earth's 

 crust. 



