368 MR BALFOUR STEWART ON EARTH-CURRENTS DURING MAGNETIC CALMS 



limited number of observations of earth-currents, and that only at one place, it is 

 surely too soon to endeavour to trace quantitatively the connection between such 

 currents and magnetic phenomena. I have therefore confined myself to a kind 

 of qualitative analysis, which perhaps may help roughly to determine the nature 

 of this connection. 



I shall be excused if I here call attention to some remarks on the subject of 

 this paper made by the Rev. Dr Lloyd of Dublin, with the view of showing that 

 the facts noticed by this eminent physicist may be explained on the induction 

 hypothesis. These remarks occur in a paper " On Earth-Currents and their 

 Connection with the Diurnal Changes of the Horizontal Magnetic Needle," which 

 is published in the Transactions of the Royal Irish Academy.* The author 

 says,— 



" When we examine the curves, in which Mr Barlow has represented the 

 course of the galvanometric deflections caused by the earth-currents, we observe 

 that the regularity of that course is continually interrupted by rapid recipro- 

 cating movements, in which the needle oscillates from one side to the other 

 of the zero alternately. These movements are similar to those of the magne- 

 tometers with which we are familiar ; but they are much more rapid, and bear 



a larger proportion to the regular changes The frequency 



and magnitude of the deflections may both be taken into account, by adding 

 together the alternate changes, without regard to sign, and dividing the sum by 

 the regular daily changes. I have selected for this calculation the obser- 

 vations made during the six hours, commencing at 3 a.m., on May 29, 1848, 

 that being a period of comparative disturbance. The sum of the changes of the 

 galvanometer needle during that period, on the Derby and Rugby line, was equi- 

 valent to 571 divisions of the instrument, — the mean daily range for the entire 

 week being 114 divisions, and the ratio = 50. The corresponding ratio for the 

 galvanometer of the Derby and Birmingham line is somewhat smaller. The sum 

 of the changes of the Greenwich declinometer during the same period was only 

 57 minutes, the mean daily range being 12-4 minutes. In like manner, the sum 

 of the changes of the horizontal force (in parts of the whole) was -0158, the mean 

 daily range being "0034. The ratio is accordingly the same for the two magnetic 

 elements, and its amount is 4*6, or less than one-tenth of the corresponding ratio 

 in the case of the galvanometric changes. We learn, therefore, that the rapid 

 changes of the earth-currents are much greater in proportion to the regular daily 

 changes, than the corresponding movements of the magnetometers." 



The fact noticed by Dr Lloyd in these remarks is quite in accordance with 

 the induction hypothesis, and is indeed only another way of expressing the truth 

 already stated, that the action of disturbances on earth currents is to increase the 



* Vol. xxiv. p. 115. 



