28 
Dr. Joule described the process he employed to harden 
steel wires for magnetic needles. The wire was held stretched 
between the ends of two iron rods bent into a semicircular 
shape. The free ends of the iron rods could be placed in 
connexion with a voltaic battery by means of mercury cups. 
Underneath the steel wire a trough of mercury was placed. 
When the ends of the iron rods dip into the cups the current 
passes through the wire, heating it to any required extent. 
When these ends are lifted the current is cut off, while at the 
same instant the heated wire is immersed in the trough of 
mercury. 
CORRIGENDA. 
Mr. Kirkman begs the reader to make the following corrections 
in the last No. of these Proceedings : — 
Page 14, line 11, for ‘ unexciting,’ read unresisting. 
„ 15, line 1 , for ‘ existence,’ read resistance. 
„ „ line 22, for ‘ forces,’ rend forms. 
„ 16, line 27, for ‘ centre,’ read centres. 
„ 17, line 2, for ‘ multiples,’ read multipliers. 
,, „ line 4, for ‘ then he contracts,’ read thus he constructs. 
„ „ line 21, for ‘force,’ read item. 
„ „ line 3 from bottom, for ‘ mere,’ road zero. 
„ 18, line 5, for ‘ vacuum,’ read volume. 
„ „ line 8, for 1 deduce,’ read adduce. 
» „ line 10, for ‘ bring,’ read any. 
