Torsion of Lead: Wire beyond the Elastic Limit, 543 
if in diameter only stretches one millimetre in the first 15 min. 
under the action of a 700 grams weight. The use of an 
- Optical lever was resorted to, and by having the telescope 
Fig. 4. 
BEAM 
\y 
x 
i = 
ra) = 
4 w 
N 
wH 
ky 
IX 
= 
. 
y 
NX 
Weal LN COPE: 
OPTICAL 
LEVER 
SYPHON 
two metres from it, it was)possible to observe changes of 
length amounting to one-taousandth of a centimetre. It 
was therefore possible to keep the wire at a fixed length 
