134 
MAJOR SABINE ON TERRESTRIAL MAGNETISM. 
the observed values of dip and intensity, according to the direction of the ship’s head 
during the observation. His memoir contains a statement of the data on which these 
Avere founded, and the observed as well as the corrected value of every observation. 
The dip results haA^e also a correction applied for the non-coincidence of the centres 
of gravity and of motion of the needle, the poles not being reversed ; this was ob- 
tained by Mr. Dunlop by comparing the results on shore, in England and Paramatta, 
with those of needles in which the complete process was gone through. I have used 
in this paper the dips as thus corrected by Mr. Dunlop himself. The intensity results 
are reduced to a common temperature. The needle which was employed to give the 
comparative values of the intensity had sustained no sensible change in its magnetic 
condition for above two years previously to the voyage, as shown by observations with 
it at Makerstown Observatory in Scotland ; nor did it undergo any in the two years 
subsequent to the voyage, as shown by observations at the Paramatta Observatory. 
That it was equally steady during the intermediate time may be inferred from the 
relative values which it gives for the total intensity at Makerstown and Paramatta. 
The times of vibration were as follows : 
S 
Makerstown, March 19, 1831 822*05 
Paramatta, December 7, 1831 622'90 
If we take for the dip and intensity at Makerstown the values observed at the neigh- 
bouring stations of Dryburgh and Melrose*, we have the dip in March 1831, allowing 
for the difference of epoch, at the annual rate of a diminution of 2 , *4'f~, 71° 50' ; and 
the total intensity expressed by (1*372 X 1*0203 =) 1*400. At Paramatta we have 
the dip from the observations of Captains FitzRoy, Bethune, and Wickham, R.N., 
as stated in the foot note|, 62° 51'. From these data we have the total force at Para- 
matta by Mr. Dunlop’s needle equal to 1*665. The value determined by Capt. Fitz- 
Roy, R.N., in 1836 is 1*685. We may conclude, therefore, that Mr. Dunlop’s needle 
suffered little or no loss of magnetism in the interval of its removal from the obser- 
vatory at Makerstown to that of Paramatta. 
I have availed myself of Mr. Dunlop’s results for the Atlantic Maps, from latitude 
6° N. and longitude 340°, where his track parted from that of Lieut. Sulivan’s, to the 
meridian of the Cape of Good Hope. The observations are not strictly accordant in 
point of epoch with the other observations represented in the map ; but the only 
quarter where this small difference of epoch ought to occasion discrepancy, is in the 
neighbourhood of the Cape of Good Hope, where, on account of the large amount of 
* Magnetical Survey of the British Islands, British Association Reports, 1838. 
t Vide the Report referred to, pp. 62 — 66. 
% Captain FitzRoy (Sydney), 1836 62 49‘4^ 
Captain Bethune (Sydney), 1837 62 52 - 7 t62°5F 
Captain Wickham (Sydney), 1838 and 1839 62 5 1 *2 J 
There has been little or no secular change of the dip at Sydney since the commencement of the present century. 
(Voyage of the Adventure and Beagle, vol. i. p. 528.) 
