LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
113 
At the Falkland Islands there were two determinations of the absolute horizontal 
intensity made by Captain Ross at the Magnetic Observatory at Port Louis, one in 
September 1842, being 6’87, and a second in November of the same year, being 6’32. 
They were both made with 15-inch magnets ; the angles of deflection were observed 
at four distances, but amounted only to 56''8, 31'’9, 21' - 4, and 1 2'*9 in the first expe- 
riment, and to 1° 49'’9, 1° 01 '’6, 41'’5, and 25'’ 1 in the second experiment. 
These values of the horizontal intensity would give that of the total intensity at 
Port Louis respectively 1’609 and l - 367- It is obvious that we can draw no con- 
clusion whatsoever from these numbers, and that we must wait for the confirma- 
tion or correction of the value given by the needles of Mr. Fox’s instrument, until 
absolute determinations can be procured with instruments capable of affording more 
satisfactory results. Steps have been taken to obtain such determinations at the 
Falkland Islands from Captain Sullivan, R.N., and at Sydney and New Zealand from 
the Surveying F^xpedition under Captain Blackwood, R.N.; when these arrive, we may 
learn whether any and what final correction will require to be applied to the intensities 
now provisionally deduced from the observations with Mr. Fox’s needles, in the Erebus 
and Terror. We may expect to receive these determinations before the time when 
the results now presented to the Royal Society will have to be combined with those 
of the preceding and succeeding years, in a general calculation of the magnetic lines 
in the southern hemisphere. 
2. With Deflectors . — In the Erebus, the spare needle R. F. 4 was employed, — as 
“ deflectorS,” with its south pole opposite to the division of the circle which the south 
pole of the mounted needle had previously indicated as the dip ; — and as “ deflector N,” 
with its north pole similarly applied to the opposite division of the circle. The angles 
of deflection varied in different localities during the voyage, in round numbers as 
follows: — Deflect. S from 52° to 71°; and deflect. N from 49° to 67°. For obtain- 
ing the equivalent weights to the deflecting force of the deflectors at these angles, 
we have the comparative observations with deflectors and weights at Hobarton, 
Sydney, New Zealand, the Falkland Islands, and on the ice in lat. —65° 47', long. 
202° 08'. The angles of deflection caused by the weights have been already stated ; 
the preceding observations ; yet from the improvement which it is natural to suppose practice must have made 
in the observers, and from the reduced discrepancies of the partial results with the smaller bars, the mean of 
the ten results in August 1843 would seem entitled to a preference over the earlier and more numerous results. 
Judging by what has been done at Woolwich with the 2’45 and 3 inch magnets, and at the Cape of Good 
Hope with 3’0 and 3‘ 67 inch, we may expect with them a still further and considerable reduction in the dis- 
crepancies of the partial results ; but it would not be safe, with the comparisons which we have now before us, 
to feel full confidence that there will be no apparently constant or systematic difference between the results of 
the larger and smaller bars. Reviewing the whole subject, we can as yet, therefore, only consider ourselves as 
being in progress towards such accuracy in determining the ratio of the intensity at different places by the ab- 
solute method, as shall be superior to that with which it was previously obtained by the employment of well- 
selected needles in relative determinations. 
Q 2 
