90 
LIEUT. COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
From these observations we have the following values of the constants : — 
Hobarton . «•=+• 0275; b= +'979 
Falkland Islands . . . a= + ’0293; £=+-994. 
These values are nearly the same as those derived from the observations in the 
Erebus at the same periods, and appear to require no special remark ; the same 
tables have been employed in the declination corrections of both ships during the 
voyage under notice ; the values of the constants in these tables were as follows : — 
a= -0267 when the ships were sailing from the lower into the higher latitudes; 
# = •0288 when sailing from the higher into the lower latitudes; &=+'984 in both 
cases. 
Deduction of the Corrections on account of the Ship's attraction for the Observations 
of Inclination. 
1. In the Erebus. — The spot in the ship in which Mr. Fox’s apparatus for the ob- 
servations of inclination and intensity was employed, was a few feet in advance 
(towards the bow), and about two feet lower in height, than the position of the 
standard compass. 
The values of a and b derived from the observations with the compass needle apply 
in strictness only to the spot in which that compass was stationed ; it may be proper, 
therefore, before we employ them for the observations with Mr. Fox’s apparatus, to 
show that nearly similar values for the constant a in particular (the more important 
constant) are deducible from the observations of inclination and intensity, independ- 
ently of those made with the compass needle. For this purpose we may employ 
equation (1.), Phil. Trans., 1843, Part II. p. 147, viz. 
^ cos & cos £'= cos 0 cos sin 0, 
obtaining by its means the value of a from the observations of inclination and inten- 
sity made at Hobarton and Port Louis. As A' is known to differ very slightly, if at 
all, from unity, we have from equation (1.), 
a sin 0=-^cos $ cos cos 0cos £. 
<p and 0 are furnished by the mean of the observations of inclination and intensity on 
the sixteen points of the compass, having approximate corrections applied to each of 
them ; <p' and O' by the (uncorrected) observations on the different points. 
From the general aspect of the observations at both stations, we may conclude that 
the same symmetrical distribution of the iron existed in reference to the position of 
Mr. Fox’s apparatus as in the case of the standard compass, and consequently that at 
the north and south points the value of and £ coincided, being equal in the one 
case to 0°, and in the other to 180°. At Hobarton (in June 1841) we have 9 = 1*83, 
0— — 70° 39' ; <p' at north 1‘81 2, at south P854 ; O' at north —71° 56', at south —69° 14' : 
