1822.] Philosophical Transactions for 1822, Part I. 371 
receive the Y’s, which raise and lower fhe needle on its sup- 
ports, and ensure that the same parts of the axis rest in each 
observation on the planes. A small brass sphere traverses on a 
steel screw, inserted in the lower edge of the needle, as nearly 
as possible in the perpendicular to the index line passing through 
the axis of motion ; by this mechanism, the centre of gravity of 
the needle, with the screw and sphere, may be made to fall more 
or less below the axis of motion, according as the sphere is 
screwed nearer or more distant from the needle, and according 
as spheres of greater or less diameter are employed. The object 
proposed in thus separating the centres of motion and gravity, 
is to give the needle a force arising from its own weighit to 
assist that of magnetism in overcoming the inequalities of the 
axis, and thus to cause the needle to return, after oscillation, 
with more certainty to the same point of the divided limb than 
it would do were the centres strictly coincident.” 
“ The centres of motion and of gravity not coinciding, the 
position which the needle assumes, when placed in the magnetic 
meridian, is not that of the dip: but the dip is deducible, by an 
easy calculation, from observations made with such a needle, 
according to the following directions : 
“Ifthe needle has been carefully made, and the screw inserted 
truly as described,” “ two observations made in the magnetic 
meridian are sufficient for the determination of the dip, the two 
faces of the needle being successively towards the observers, 
renewing the position of the axis on its supports in such a man- 
ner that the edge of the needle which is uppermost in the one 
observation becomes lowermost in the other; the angles which 
the needle makes with the vertical in these two positions being 
read, the mean of the tangents of those angles is the co-tangent 
of the dip. But when needles are used in which this adjust- 
ment has not been made, or where its accuracy cannot be relied 
on, four observations are required ; two being those which are 
already directed ; the two others are similar to them, but with 
the poles of the needle reversed; calling then the first arcs F 
and f, and those with the poles reversed G and g, and taking 
tang. F + tan.f = A 
tang. F — tan. f = B 
tang. G + tan. g = C 
tang. G — tan.g = D 
— + aa = twice the co-tangent of the dip,” 
“The instrument in which the needle was tried is already 
described in the Philosophical Transactions for 1819, p. 132, and 
several improvements which have since been added, in the 
see to Capt. Parry’s Voyages of Discovery, pp. 107, 159, 
iC. 
«The experiments were made in the nursery-garden in the 
232 
