amount of the dip of the magnetic needle , in London, &c. 7 
The demonstration of this formula is given in the treatise 
which has been referred to ; the investigation is simple. 
In reversing the poles, it is not necessary that the magnetic 
force imparted to the needle should be the same in amount as 
it possessed previously to the operation. 
By adopting the precaution of placing the needle in a 
groove to prevent its lateral motion, and by confining the 
sides of the magnet by parallel strips of wood, so that in 
moving along the needle they may preserve its direction, the 
poles may always be ensured to coincide with the extremities 
of the longitudinal axis. 
It is desirable to be furnished with several spheres of 
different diameters, to possess the means of proportioning the 
force, arising from the eccentricity, to the force of mag- 
netism, as the latter is doubled between the equator and the 
poles ; it may be expedient that the force of magnetism 
should always predominate, though no such inference ap- 
pears to result from the present experiments, in which eight 
spheres were used, numbered according to size. No. 1 being 
the largest. 
If the distance between the centres of motion and of gravity 
be considerable, the arcs in the alternate observations will be 
on different sides of the vertical, especially where the dip is 
great ; in such cases the arcs to the south of the vertical are 
read negatively. 
The instrument in which the needle was tried is already 
described in the Philosophical Transactions for 1819, page 
132, and several improvements which have been since added, 
in the Appendix to Captain Parry’s Voyage of Discovery, 
pages cvii. cxxxix., &c. ; the perfect horizontality of the 
