168 
LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
Captain Ross commenced the experiments for measuring 1 the absolute horizontal 
intensity at Hobarton, by obtaining five results with the large magnets of his obser- 
vatory magnetometers on different days whilst the Expedition was refitting there. 
The details of these will be published with the other magnetometric observations of 
the voyage ; the results, which have been computed by Lieutenant Goodenough of 
the Royal Artillery from the data received from Captain Ross, are as follows : — 
October 13, 1840 . . . 
. . 4*491] 
May 3, 1841 
. . 4*626 
May 21, 1841 ...... 
. . 4*602 \ 
June 5, 1841 
. . 4*579 
June 25, 1841 
. . 4*566, 
Mean 4-573*. 
The dip being — 70° 40' at Hobarton, and the approximate value of the absolute 
horizontal intensity at Woolwich 3-72-^ with the dip of 69° 03', the corresponding 
value of the total intensity at Hobarton in the arbitrary scale (London = T372) is 
1*821. The previous relative observations, collected in No. I. of these Contributions, 
had given 1*819 as the mean of three determinations, viz. 
FitzRoy . . . . 
Franklin and 
Sabine 
Wickham. . . . 
1836 
1837 
1838 
1838 
1*817' 
T810 i- Mean T819. 
T830 J 
The closeness in the accordance of the mean results by the two methods can only 
be viewed as accidental, because the probable error of the absolute determination, 
estimated from the differences in the partial results, is far greater than the difference 
of the two methods; but it fully warrants 1*82 being now taken as a provisional 
value of the total intensity at Hobarton, as the base station of the observations 
which form the subject of this number, regarding 1*82 + e as the true value, and e as 
a small correction to be determined hereafter, applicable to the whole series. 
The weights employed in deflecting the intensity needle were from half a grain to 
six grains. It was soon found that half a grain was too small to give satisfactory re- 
sults, and observations with that weight were discontinued. I have not therefore 
taken the observations made with it into account, except at Hobarton, where they 
* Since these pages were written I have received the results of twenty-two monthly determinations of the 
absolute horizontal intensity at the magnetic observatory at Hobarton (ten in 1841 and twelve in 1842) made 
and computed by Lieutenant Kav, R.N., and the naval officers under his direction. The mean in 1841 is 
4 - 553, the partial results varying from 4‘601 to 4'509 ; and the mean in 1842, 4 - 513, the partial results vary- 
ing from 4'443 to 4 - 568. The discordance in the partial results of these observations is scarcely less than in 
those of Captain Ross : there is also a considerable disagreement in the means of the three series, which may 
not improbably be diminished when the particulars of the several observations shall have been carefully ex- 
amined, though the partial results must still be expected to differ much more widely than could be desired. It 
is hoped that such differences will be reduced within much smaller limits by the use of the improved apparatus 
which has recently been supplied to the Hobarton as well as to the other colonial observatories, 
t Philosophical Transactions, 1843, Art. X. 
