MAJOR SABINE ON TERRESTRIAL MAGNETISM. 
133 
sponding to the observation. The eastern track in the northern Atlantic and the 
western in the southern Atlantic are those of the outward voyage in the Arrow. 
In the Archives of the Royal Society is an account by Mr. James Dunlop, Astro- 
nomer of the Observatory at Paramatta, of observations on the magnetic dip and 
horizontal intensity made on board the merchant ship in which he took his passage 
from England to Australia in 1831*. These observations were repeated daily, with 
very few exceptions, from the middle of June to the middle of October. The dipping- 
needle employed was twelve inches in length, with a circle graduated to 20'. The 
needle was suspended in the centre of the circle by a small frame of brass, having 
jewelled holes for the terminations of the axle, and turning freely in azimuth, by the 
directive force of the needle itself, upon two fine brass points, one above and one below, 
working also in jewelled holes. The whole instrument was suspended by hooking in- 
verted Y’s, attached to its upper part, on two cylinders projecting from a very fine uni- 
versal joint screwed to the roof of Mr. Dunlop’s cabin. The universal joint was of rings, 
and could be turned on the centre, by the hand, in azimuth, to accommodate the plane 
of the instrument to that of the magnetic meridian indicated by the needle. The dip 
was observed by taking a mean of the extreme arcs of the small oscillations produced 
by the motion of the ship. The whole was inclosed in plate glass in the usual manner. 
The intensity was observed by an apparatus similar to M. Hansteen’s but larger, 
the box being about eight inches in diameter, and three inches in depth, and the tube 
over the centre, from which the needle was suspended by the silk fibre, was about 
nine inches in length. In each of the four corners of the upper side of the box was 
screwed a small, brass pin with a hole in it, into which were hooked the ends of four 
copper wires which terminated in a small brass ring about -fth of an inch in diameter 
over the centre of the tube, and as near the point of suspension of the needle as pos- 
sible, in order that the needle and box might be suspended nearly from the same 
point, and also in the same line with the centre of gravity of the box. A brass wire 
of about -^vth of an inch in diameter was screwed about an inch into the deck, the 
lower end terminating in a small circular hook about the eighth of an inch in dia- 
meter, on which the box was suspended by the small ring on the vertex of the copper 
wires, forming a delicate universal joint. 
The instruments for the dip and intensity were suspended as nearly as possible 
from the same part of the deck ; no alteration was made in the mode of their sus- 
pension during the voyage : the trunks in the cabin contained nothing which could 
affect the needles, and no change was made in their disposition. 
By embracing every opportunity at sea which a calm afforded of making observa- 
tions with the ship’s head on various points of the compass, or when alterations in the 
course took place in the same day, Mr. Dunlop obtained corrections to be applied to 
* I was not aware of the existence of these observations when the report “ on the variations of the magnetic 
intensity at different parts of the earth’s surface” was drawn up, or they would have been included in that 
memoir. 
