and form for a compass needle. 117 
The ends of the needles were now hardened at a red heat, 
the middle remaining soft, as before. 
Experiment 16. 
Needles hardened at the ends at a red 
heat. 
Directive 
force. 
Parallelogram 
Rhombus 
Pierced rhombus - 
1020 
1185 
Experiment 17. 
Ends hardened at a bright red. 
Directive 
force. 
Parallelogram 
Rhombus 
Pierced rhombus 
1258 
97 ° 
IO85 
The ends hardened at a red heat as near to that employed 
in Experiment 16 , as possible. 
Experiment 18. 
Ends hardened at a red heat. 
Directive 
force. 
Parallelogram 
Rhombus 
Pierced rhombus 
1350 
1121 
1205 
Here it should seem that an increase of power has been 
