137 



We possess, in addition to the foregoing, similar results at Lake Atha- 

 basca, in British IS"orth America, deduced by Colonel Lefroy from obser- 

 vations made by himself, and which, although derived from a shorter 

 series of observations, are of the highest scientific value. For these 

 places, therefore, it only remains to combine the results of the decli- 

 nation and horizontal intensity, by the method which has been already 

 applied to the regular changes of the same elements. 



The result of this calculation, applied to the Dublin observations, 

 shows that the direction of the disturhance-current at that place observes 

 a mean law, not very dissimilar to that which governs the regular diurnal 

 current. Its azimuth rotates, during the day, in the same direction as 

 the sun, its direction pointing almost exactly to the luminary. The 

 direction is east about 5 a. m. ; south, about noon ; and west, at 6 p. m. 

 The current is easterly from 9 p. m. to 9 a. m., inclusive, and westerly 

 during the remainder of the 24 hours. The mean azimuth of the easterly 

 current, measured from the north eastward, is 40° 15^; that of the 

 westerly is 230° 18^ If the mean directions of the easterly and west- 

 erly currents be assumed to be in the same right line, the mean azimuths 

 will be 45° E., and S. 45° W. This result agrees, in a very remark- 

 able manner, with those obtained by Mr. Barlow and Mr. Walker from the 

 direct measures of the intensity of the Earth-currents, as observed on 

 days of disturbance in several of the telegraphic lines of England ; and 

 the agreement must be regarded as an additional proof of the dependence 

 of the magnetic changes upon Earth-currents. 



The phenomena at Makerstoun are very similar to those at Dublin ; 

 and the epochs of the passage of the current through the cardinal points 

 are nearly the same. 



At Toronto, in Canada, the current is wholly easterly, the mean azi- 

 muth being 81° 25'. On the other hand, at Athabasca, the current is 

 easterly from 12 p. m. to 6 a.m., inclusive, and westerly during the re- 

 mainder of the 24 hours ; the sums of the easterly and westerly changes 

 for the entire day balance one another, the easterly currents being as 

 much greater in magnitude as they are less in duration. The mean 

 azimuths are 110° 18' and 290° 56'. 



At St. Helena the direction of the current is easterly throughout the 

 day, the mean azimuth being 70° 53'. The direction is singularly con- 

 stant, the greatest deviation from the mean being only 10°. The phe- 

 nomena at the Cape of Good Hope closely resemble those at St. Helena, 

 The direction of the current is easterly at every hour, excepting 5 a.m.. 

 when there is a slight westerly movement. The mean azimuth is 

 77° 54^ 



It thus appears that at some places — as in the British Islands — the 

 mean direction of the disturbance current rotates through the entire 

 compass in the course of the day ; while at others — as Munich, Toronto, 

 St. Helena, and the Cape of Grood Hope — it is easterly throughout the day. 

 While, therefore, there is a periodicity in the easterly and westerly cur- 

 rents depending on the hour, we are obliged to infer that there is, at the 

 same time, some cause constantly operating which tends to produce an 

 easterly current. 



