the Royal Society’s Injlruments* 399 
if the needle vibrates in arches of five or more degrees, 
when let down on the planes, there will frequently be as 
great an error in the dip. It is true, that the part of the 
agate planes, which the axis refts on when the vibrations 
are flopped, will be a little different according to the 
point which the needle flood at before it was let down ; 
which will make a fmall difference in the dip as fliewn 
by the divided circles, when only one end of the needle 
is obferved, though the real dip or inclination of the 
needle to the horizon is not altered : but this difference is 
by much too fmall to be perceived ; fo that the above- 
mentioned error cannot be owing to this caufe. Neither 
does it feem owing to any irregularity in the furface of 
the agate planes, for they were ground and polifhed with 
great accuracy; but it moft likely proceeds from the axis 
flipping in the large vibrations, fo as to make the agate 
planes bear againft a different part of it from what they 
would otherwife do. I have great reafon to think, that 
this irregularity is not owing either to want of care or 
fkill in the execution, but to the unavoidable imperfec- 
tion of this kind of work. I imagine too, that this inftru- 
ment is at leaft as exa£t, if not more fo, than any which 
has been yet made. 
The following table contains, the refult of fome ob- 
fervations which I made, partly with a view to determine 
the true dip at this time in a place out of reach of the 
influence of any iron work, and partly to fee how nearly 
different needles would agree. The inftruments were all 
tried in the fame garden in which the variation compals 
was obferved, and all on the 10th, 1 ith, 1 3th, and 14th 
