396 Mr. cavendish’s sic count of 
to the Weft, and then to the Eaft; after which the poles 
of the needle were reverfed, and the dip obferved both 
ways as before. The reafon of this is, that the mean of 
the obferved dips, in thefe four lituations, differs very 
little from the truth, though the needle is not well ba- 
lanced, and even though a great many other errors are 
committed in the conftruction of the inftrument; pro- 
vided the needle is made equally magnetical after the 
poles are reverfed as before W; and that the difference of 
the obferved dip, in thefe four lituations, is not very 
great, as will appear from the following confiderations. 
Firft, let fig. 7. be a front view of the needle; ab a 
line parallel to the direction of magnetifm therein ; and 
cd a perpendicular thereto, meeting it in the line joining 
the centers of the cylindrical ends of the axis, or in the axis 
of motion as we may call it. If the needle was truly ba- 
lanced, its center of gravity would be i nd, the interfedlion 
of ab and cd. Suppafe now, that the needle is not truly 
balanced, but that its center of gravity is in g; draw gn 
perpendicular to ab, cutting it in m\ and let the parts 
mn and mg be equal. When the inftrument is turned 
half-way round, fo that the contrary face of the needle 
is prefented towards us, the edge adb, which is now 
loweft, will become uppermoft, and the center of gravity 
will be in that fituation in which the point n now isj 
therefore, the mean between the forces with which the 
(d) It is eafy to fee whether the needle is- made equally magnetical after 
the poles arc reverfed as before, by counting the number of vibrations which it 
makes in a minute. 
I 
needle 
