and form for a compass needle. 
i li 
Experiment 7. 
Hardened with a bright red heat, and then sof- 
tened by a red heat from the middle towards 
the ends, the extremities for about an inch 
remaining hard. 
Directive 
force. 
Parallelogram 
6ll 
Small rhombus 
710 
Large rhombus 
660 
Pierced rhombus 
685 
Experiment 8. 
Softened at a red heat between two plates of 
steel, the whole being allowed to cool gradu- 
ally, end then the extremities of ihe needles 
hardened at a red heat. 
Directive 
force. 
Parallelogram 
520 
Small rhombus 
585 
Large rhombus 
554 
Pierced rhombus 
590 
As it appeared from the above experiments that the needles 
had suffered a gradual deterioration, I imagined that this might 
have occurred in consequence of their having been exposed to 
the heat of a coal fire, by which some portion of the carbon of 
the steel might have been destroyed ; I therefore re-carboni- 
zed the needles, by surrounding them with shreds of leather, 
and exposing them for several hours in a dose vessel to a 
considerable heat. After they had gradually cooled, the ends 
were hardened at a red heat. 
