274 ON EARTH-CURRENTS, AND THE DIURNAL 



vatory, and consequently at the same intervals of time as the 

 needles of the galvanometers in Mr. Barlow's observations. I find, 

 upon an examination of the two records, that there were on that 

 day, on the average, 5-1 alternations of the galvanometer needles in 

 the hour, the corresponding number of alternations of the magneto- 

 meters being only 3-1. The numbers are very nearly the same for 

 the two galvanometers of the two lines of telegraph, as well as for 

 the two magnetometers of the Greenwich Observatory. And the 

 disproportion in the magnitude of the deflections, as compared with 

 the amount of the regular daily changes, appears to be even more 

 considerable. 



The frequency and the magnitude of the deflections may both be 

 taken into account, by adding together the alternate changes, with- 

 out regard to sign, and dividing the sum by the regular daily 

 changes. I have selected for this calculation the observations 

 made during the six hours commencing at 3 A. M. on May 29, 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 equivalent to 571 divisions of the instrument, the 

 mean daily range for the entire week being 11 "4 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 there- 

 fore 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 physical interpretation of the foregoing facts will be con- 

 sidered in the sequel. For the present, I shall merely notice the 

 consequence which appears to flow immediately from them namely, 

 that little or no accordance is to be expected, in comparing the 

 individual remits of the two classes of measures. I have verified 

 this conclusion by a comparison of Mr. Barlow's diagram, for 

 the period above referred to, with the corresponding Greenwich 

 observations. 



