I 
ler. 
And having 
( ^5>8 ) 
bended the Wirein this FoniQ, 
I marked one End thereof, and drove into the Hole 
a final! brafs Pin fitted to it, which was very round 
and fharp at the Point, which refted on a deep Piano- 
Concave Lens of Glafs well polilhed. (See the Fi^ 
gure.) I fitted a Box for it with a Glafs over it^ 
which Glafs was faftened with a Ring of brafs Wire, 
as the Glafles of Telefcopes are which Ring- kept out 
Air, otherwife had been needlefs. The Glafi Concave 
was fixed in the great End of a thin Brafs Ferule (like 
that off a Staff) juft fit for it, and the fmall End of 
the Ferule was fixed in a Hole made for it in the Mid- 
dle of the Bottom of the Box : I alfo put a Ring of 
thin Brafs on the Top of the Lens, not only to keep 
it in fteady, but to prevent the Pin from going in be- 
twixt the Lens and the Ferule, which fpoilsits Point. 
Doubtlefs a Concave of Diamond is much better. 
Whenfoever I ufed one of cither Sort of thefe Nee- 
dles (efpecially for fuch Experiments as required it to 
be perfeftly void of fixed Polarity) I was obliged to 
keep it in a Motion either librating up and down like 
the Beam of a Pair of Scales, or trembling (which is 
a ftiort pendulous Ofcillation from Side to Side) or elfe 
both librating and trembling at the fame Time; which 
faid two Motions being at right Angles with each other, 
are not inconfiftent : And if the Needle is truly poifed, 
the horizontal Verticity is neither obftrufted nor ac- 
celerated by the Librations, becaufe they are at right 
Angles thererewifh, nor by the Tremblings, becaufe the 
two Ends perfedly balance one another in contrary Mor 
tion,. 
