1863.] 629 



upon the question of annual variation, may be briefly stated as 

 follows : 1st. The dip is subject to an annual variation, which, on 

 the average of the six years, amounts to 1''35 ; consisting of a semi- 

 annual inequality with epochs coinciding, or very nearly so, with the 

 equinoxes ; the mean dip being on the average 0'*65 lower than its 

 annual mean value in the six months from April to September, and 

 0'*7 higher than its annual mean value in the six months from 

 October to March. 2nd. That the horizontal force is subject to 

 a semiannual inequality having the same epochs being on the 

 average '0013 higher than its annual mean in each of the six months 

 from April to September, and '0013 lower than its annual mean in 

 each of the months from October to March. 3rd. That, combining 

 the results of the dip and horizontal force, the total terrestrial 

 magnetic force is expressed in British units by 10'3002 as its mean 

 value in the months from April to September, and by 10-30347 in 

 the months from October to March, there being thus a difference 

 of -00327, by which the intensity of the magnetic force of the earth 

 is greater in the months when the sun is south of the equator than 

 in the months when he is north of the equator. 



This conclusion is compared with the results obtained in a corre- 

 sponding manner from the published observations of the Hobarton 

 Observatory, viz., with the monthly determinations of the horizontal 

 force in the five years from January 1846 to December 1850 inclu- 

 sive, and with those of the dip in the ten years from January 1841 

 to December 1850 inclusive. From these data the conclusions are 

 drawn, 1st, that at Hobarton the dip is subject to an annual variation 

 amounting to l'-18, consisting of a semiannual inequality with 

 epochs coinciding or nearly so with the equinoxes the (south) dip 

 being on the average 0''59 less in the months from April to Sep- 

 tember, and 0''59 greater in the months from October to March than 

 the mean annual value j and 2nd, that the horizontal force is subject 

 to a similar semiannual inequality, being *0007 less than its mean 

 value in the months from April to September, and '0005 greater in 

 the months from October to March ; and combining these two 

 results, that the total force at Hobarton is expressed in British 

 units by 13-56882 in the months from October to March, and by 

 13*55195 in the months from April to September : the difference, 

 01687, expresses the measure of the greater intensity of the earth's 



VOL. XII. 2 Y 



