1863.] 627 



towards the east when the sun is north of the equator, and to the 

 west when he is south of the equator. The amount of the deflection 

 in the first-named semiannual period is, on the average, 19"'l in 

 each week; and in the opposite semiannual period 19"; making 

 together an annual variation of 38"' 1. 



The author then refers to the result of a similar investigation 

 of the phenomena at St. Helena in the equatorial zone, the parti- 

 culars of which have been already published in the 2nd volume of 

 the St. Helena Observations. The result, derived from eight years 

 of observation, of which five years were hourly, evidences at that 

 station also the existence of a semiannual inequality with epochs 

 coinciding, or nearly so, with the equinoxes the deflections being also 

 in the same directions as those at Kew and Hobarton, viz., to the 

 east when the sun is north, and to the west when he is south, of the 

 equator. The amount of the annual variation thus produced is less 

 at St. Helena than at either Kew or Hobarton the semiannual dif- 

 ference being about 7'', and the annual variation 14". 



The author remarks that the difference in the amount of deflection 

 at the three stations may, in part at least, be occasioned by the differ- 

 ence in amount of the antagonistic force of the earth's magnetism, 

 tending to retain the magnet in its mean position in opposition to all 

 disturbing causes. The antagonistic force, viz. the horizontal com- 

 ponent of the earth's magnetic force, is approximately 5*6 (in British 

 units) at St. Helena, 4'5 at Hobarton, and 3'8 at Kew. 



In a note appended subsequently to the delivery of this paper, viz. 

 on June 19, 1863, the author refers to a similar investigation of the 

 phenomena at the Cape of Good Hope, published in 1851, in the 1st 

 volume of the magnetical observations at that station. The volume 

 contains the fortnightly means of the declination from July 1842 to 

 July 1846, corrected for secular change, and collected in Table III., 

 page v, of that volume. The differences of the declination in each 

 fortnight, so corrected, from the mean declination of the whole period, 

 are shown in its final column. The mean of the thirteen fortnights 

 (in the four years) between March 26 and Sept. 23, is O ft 4 more 

 easterly y and of the thirteen fortnights between September 24 and 

 March 25, 0''4 more westerly than the mean value showing an 

 annual variation of O f> 8 (or 48"), or a semiannual inequality ave- 

 raging 24" to the east in the thirteen fortnights from March 26 



