380 
LIEUT.-GENERAL SABINE ON TERRESTRIAL MAGNETISM. 
Table in Contribution No. VIII. pp. 352, 353 ; and the same was done for the third year 
of the Survey from a mean of similar observations at Port Louis in the Falkland Islands 
in August 1842, and at the Cape of Good Hope in April 1843, as stated in Contribution 
No. X. pp. 455-457. The corrections for the Deviation in the observations of the 
Inclination and of the Intensity were computed in all cases by the method described in 
Mr. Smith’s Memorandum in the present Number, page 379, — employing for the first and 
second year’s Surveys the mean of the observations of the ‘Erebus’ at Hobarton in October 
1840, before her departure for the south, and at the same place, in June 1841, on her 
return*, and for the third year from the ‘ Erebus ’ Table in Contribution X. p. 459, and 
the ‘Terror’ Table in Contribution X. p. 461. 
The Deviations of the Declination became very large when the Inclination approached 
90° ; in 88° of Inclination, for example, the influence of the ship’s iron occasioned a 
deviation amounting to the excessive value of 50° and upwards at the extreme points ; 
so that an observation with the ship’s head East would in consequence differ 100° or 
even more from an observation taken at the same place with the ship’s head West. 
It was doubtless the experience of such extreme differences which led to what might 
almost appear an excessive multiplication of observations, as well as to the constant 
endeavour to diversify the circumstances of the direction of the ship’s head, both 
which precautions have contributed in no small degree to reduce within such com- 
paratively small limits the differences which are seen in the Tables between the 
mean values of the Declination on the same or on successive days. Even the 
remaining minor differences which appear might probably have been still further 
reduced by another precaution, the desirability of which may not have been so clearly 
apprehended at the time, namely, the recording the direction of the ship’s head at the 
precise instant when the pointing of the compass is read. Unless the two observed 
facts strictly synchronize, the very slight and almost momentary change in the direction 
of the ship’s head will occasion an apparent discordance in successive results when the 
ship’s head is near the extreme points of East or West, though it may have little ulti- 
mate influence on the position or the direction of the isogonic lines in the final result 
of a great body of observations. This remark applies in a far minor degree to the differ- 
ences of successive or of nearly adjacent results of the Inclination or Intensity, where 
no such excessive increase in the amount of the deviation-corrections takes place when 
approaching the point of 90° of Inclination. A critical eye may well regard the general 
* Mr. Pox’s apparatus, designed for the use of the c Terror ’ in the Antarctic Survey, not having been quite 
ready when the ‘ Erebus ’ and ‘ Terror ’ sailed from England in 1839, a smaller apparatus was supplied for 
immediate use ; and the apparatus actually employed (in the 1 Terror ’) in the Survey did not reach that ship 
until she was on the point of sailing from Hobarton in 1840. It was found, however, on careful subsequent 
examination that in the two ships the corrections for the deviations of the Inclination and of the Porce were 
so nearly the same in the course of the first two years of the Survey, that the Table computed from the obser- 
vations of the £ Erebus ’ referred to in the text might safely be employed for the correction of the observations 
of these elements in the ‘ Terror.’ 
