464 LIEUT.-GEXERAL SABINE ON TEERESTEIAL MAGNETISM. 



Absolute Value in British Units of the Total Magnetic Force at the Cape of Good Hope 



Magnetic Observatory. 



The experiments made at the Cape of Good Hope Observatory for the determination 

 of the absolute value of the total force in British units at the time of the southern 

 survey, vi'ere published in 1851 in vol. I. of the " Magnetical and Meteorological Obser- 

 vations at the Cape of Good Hope." Tables XXXV. and XXXVI. in pages Ixiii to 

 Ixx of that volume, contain the details of thirty-five monthly determinations of the 

 absolute horizontal force, extending, with occasional interruptions, from November 1846 

 to February 1850, giving as a mean result 4-4969 at the mean epoch of July 1848, 

 and -0061 as the rate of annual secular decrease between March 1846 and February 

 1850. Hence we obtain 4'5335 as an approximate value for the middle of the year 

 1842. Table XIX. p. 1 of the same volume exhibits the mean results of fifty-eight 

 monthly determinations of the Inclination, extending from June 1841 to March 1846, of 

 which the full details are given in pages 394 to 407. The arithmetical mean is 

 — 53° 21'*1, corresponding to Nov. 1, 1843, with a mean secular increase of south dip 

 in each year of 5'"45 ; whence the approximate inclination corresponding to the middle 

 of 1842 is -58° 13'-85. We thus obtain 4-5335 x sec. 53° 13'-85=7-5736 as the 

 approximate value of the total force in British units at the Cape Observatory, corre- 

 sponding to the middle of 1842. 



Simon's Bay, the anchorage of the ' Erebus' and ' Terror' in 1843, is about fifteen geo- 

 graphical miles south of the Cape Observatory, a difference which, in conformity with the 

 maps of the isodynamic lines in that vicinity, may be regarded as equivalent to a differ- 

 ence of +0-024 of the Force, which, applied to the result at the observatory, gives 7-598 

 as approximately the total force at Simon's Bay in the middle of 1842. 



Comparison of the results given by the needles employed in determining the ratios of the 

 force in the sea observations, with the absolute values at Hobarton and the Cape of 

 Good Rope. 



The ratios of the force shown by these needles are measured by the angles of deflection 

 produced in different localities by a constant weight applied to a grooved wheel attached 

 to the axle of the needle ; the intensity of the magnetic force being inversely as the 

 sines of the angle of deflection. If we express by <p the absolute value of the force at a 

 base station, and by v the deflection caused by a constant weight at the base station, and 



by (p' and v' corresponding values at another station, we have (p'=^ ^~»?' aii<i t^J^ing 



Hobarton as the base station, we have <p'=13-540 -. — ,. The weights employed were 



smtr o 1. ^ 



grains and half grains, several of each having been carefuUy prepared by Mr. Fox him- 

 self, and the same individual weights being, as far as possible, used throughout. The 

 deflections caused by the different weights when the needles of the ' Erebus' and ' Terror' 

 were observed at the magnetic observatory at Hobarton in April 1841, are shown in the 



