39° A/h cavendish’s Account of 
into vibration, which caufed great trouble to the obferver. 
This inconvenience is intirely removed by the prelent 
conft ruction. There is no other material difference, ex- 
cept that of the needle being made to invert, and the ad- 
dition oi. the telefcope. The contrivance of fixing the 
pin which carries the needle to the lower frame, is taken 
j * 
from an inftrument of Lord Charles cavendish; that 
oi making the needle invert I have feen in fome conn 
paffes made by sisson. 
There is a very common fault in the agate-caps ufually 
made tor needles, which is, that they are not hollowed to 
a regular concave, but have a little projecting part in the 
center of the hollow ; the confequence of which is, that the 
point of the pin will not always bear againft the fame part 
of the agate, and confequently the needle will not always 
hand horizontal; but fometimes one end will Hand high eft, 
and fometimes the other, which caufes a difficulty in ob- 
ferving. There is alfo another inconvenience attends it 
when the indices of the needle are on a level with the 
point of the pin, which is of more confequence; namely, 
that it caufes the two indices not to agree, and confe- 
quently makes a fenfible error, when only one index is 
made ufe of, at leaft in nice obfervations : but when the 
lines, ferving by way of index, are drawn on the needle 
itfelf, and therefore are nearly on a level with its center 
of gravity, it can caufe very little error. The agate cap, 
which was firft made for this iiffirumcnt, was of this 
kind ; and was fo faulty, that, if no better could have been 
procured, it would have been neceHary either to have 
5 drawn 
