39 8 Afr. cavendish’s Account of 
the fame horizontal plane with it. If, indeed, the axis of 
motion is not in the fame horizontal plane with the center 
of the divided circle, the error proceeding from thence 
will not he compenfated by this method of obferving, 
unlefs both ends of the needle are made ufe of. This, 
however, is of no cbnfequence as, it is eafy to examine 
whether they are in the fame horizontal plane or not. 
But the error which is mod: difficult to be avoided is, 
that which proceeds from the ends of the axis being not 
truly cylindrical. I before faid, that the parts of them 
which reft on the agate planes are always exactly the 
fame. The inftrument is fo contrived, however, that we 
may or occafion, by giving the axis a little liberty in the 
notches by which it is lifted up and down, make thofe 
planes bear againft a part of the axis diftant about 
or -^th of an inch from their ufual point of bearing. 
Now, I find, that when the axis is confined fo as to have 
none of this liberty, and when care is taken, by previoufly 
making the needle hand at nearly the right dip, that it 
fhall vibrate in very fmall arches when let down on the 
planes; that then, if the needle is lifted up and down 
any number of times, it will commonly fettle exa£tly at 
the fame point each time, at leafl the difference is fo fmall 
as to be fcarcely fenfible; but if it is not fo confined, 
there will often be a difference of 2 o' in the dip, ac- 
cording as different parts of the axis reft on the 
planes, and that though care is taken to free the axis and 
planes from duft as perfectly as poffible, which can be 
owing only to fome irregularity in the axis. Moreover, 
if 
