LIEUT.-GENERAIi SABINE ON TERRESTRIAL MAGNETISM. 463 



to be extremely small, and unless in cases when a more than ordinary accuracy is desired, 

 the correction on this account may be regarded as insignificant. The third correction, 

 or that for any notable change in the magnetism of the needle which may take place 

 from time to time, may present greater difficulties than either of the two others, inas- 

 much as when such change has been shown to have occurred in the interval (sometimes 

 of considerable length) between the comparisons made and repeated at base stations, 

 it may not be always possible to assign the precise date at which the change commenced 

 or terminated, or the proportions in which it should be allotted to different portions of 

 the interval. It is always therefore extremely satisfactory to find, as will be shown to 

 have been the case in the ' Erebus ' and ' Terror,' that the intensity-needles preserved 

 their magnetism absolutely without sensible change throughout the interval, i. e. in the 

 present case from the time of their departure from Hobarton in April 1841 until their 

 arrival at the Cape of Good Hope in April 1843: the investigation by which this is 

 shown is subjoined ; pp. 464 and 466. The correction of the sea observations for the 

 influence of the ship's iron is subsequently discussed; viz., in p. 474. 



Absolute Value in British Units of the Total Magnetic Force at the Hobarton Magnetic 



Observatory. 



The experiments which were made at the Hobarton Magnetic Observatory for the 

 determination of the absolute value of the total magnetic force in British units in the 

 yeai's when the Southern Survey was in progress, were (1) those of the absolute hori- 

 zontal force, of which a fully detailed account was published in the first volume of the 

 " Magnetical and Meteorological Observations at the Hobarton Observatory," printed in 

 1850, pp. 381-393 ; and (2) those of the Inclination, of which an also fully detailed 

 account may be referred to in pp. 332-349 of the same volume. 



For the horizontal force we find, in the preliminary discussion prefixed to the observa- 

 tional details in that volume, at p. xxxix., a summary of 399 results obtained by Captain 

 Kay, R.N., and his assistants, with seven magnets of different lengths, between August 

 1843 and December 1848, of which the arithmetical mean is 4-4895 in British units, 

 corresponding to about the middle of the year 1846. The mean secular change derived 

 from a consecutive series of thirty-six months with the magnet which appeared to be 

 entitled to the most dependence, was an annual decrease of -0027: we have therefore 

 4-5000 as the absolute value of the horizontal force corresponding to the middle of the 

 year 1842. 



In the same preliminary discussion (p. Ixxiii) the Inclination derived from eighty- 

 seven monthly determinations between 1841 and 1848 is stated to be —70° 35'-6, corre- 

 sponding in epoch to May 1845 ; and as the annual secular change of the Inclination at 

 Hobai-ton at the period in question had been found not to exceed a small fraction of a 

 minute, the same value may be taken approximately as applicable to the middle of the 

 year 1842. 



Hence we obtain 4-500 X sec. 70° 35'6 = 13-540 as (approximately) the total force 

 in British units at the magnetic observatory at Hobarton in the middle of 1842. 



3t2 



