METHOD OF DRAWING FINE WIRES. 
845 
For the purpose of removing the coating of silver that sur- 
irouncis it, the wire must be steeped fora few minutes in warm 
initrous acid, which dissolves the silver without danger of doing 
any injury to the gold. And though it might be diiHcult in 
this manner to preserve any considerable length of such wires, 
i it is of little importance for any of those uses to which it is 
1 likely to be applied. 
Jn my endeavours to make slender gold wires by the method 
I jabove described, the difiiculty of drilling the central hole in 
ja metal so tough as fine silver, was greater than I had exjxjcfed, 
, jand I was induced to try whether plalina might not be sub- 
sstituted for the gold, as in that case its infiisibdity would allow 
ime to coat it with silver without the necessity of drilling. 
Having formed a cylindrical mould one third of an inch in Vci v fi;K \wre 
I idiameter, I fixed in the centre of it a plalina wire previously 
1 >drawn to the of an inch, and then filled the mould with 
<isilver When this rod was drawn to -y'-, mv plalina was re- 
I educed to and by successive reduction 1 obtained wires of 
and each excellent for applying to the eye-pieces 
I cof astronomical instruments, and perhaps as fine as can be 
I luseful for such purposes*. 
Since this had been the primary object that I had in view, 
11 should have thought my time ill bestowed in pursuing far- 
! tther the practical application of a method to which there seems 
I :no limit, except the imperfections of the metal employed. 
! IBut as I found by trial the tenacity of these wires to be greater 
than was to be expected in proportion to their substance, that 
! ^circumstance excited some doubts regarding the correctness of 
I the estimate by which their diameter had been deduced. 
(Other wires were consequently drawn with the utmost care, 
I las to the quality and substance of the platina employed, and 
I .as to the proportional reduction of its diameter in the process 
I of wire-drawing. 
• No vprv accurate observations can be made with a telescope 
' shorter than thirty inches, and at that distance ^ s«T of an inch sub- 
tends only one second of a degree. 
The 
