and form for a compass needle. 
"9 
Experiment 22. 
Softened throughout to a greyish white, 
the ends hardened at a red heat. 
Directive 
force. 
Rhombus 
Pierced rhombus 
1025 
H85 
Experiment 23. 
Hardened throughout, and then softened 
to a greyish white, as in Experiment 2 1 . 
Directive 
force. 
Rhombus 
Pierced rhombus 
1065 
1 l8o 
This last series of experiments presents a curious circum- 
stance. From the experiments made by Coulomb, as well as 
from the general tenor of my own, the rhombus is found ca- 
pable of receiving a greater directive energy than the parallel- 
ogram ; yet here we perceive that the parallelogram, though 
formed of the very same plate of steel as the other needles, 
is not only under every circumstance superior to the rhombus, 
but also to the pierced rhombus. It is difficult to form any 
plausible conjecture as to the cause of this difference. 
The weight of the rhombus in Experiment 10, made of 
clock spring, was 63 grains ; that made of saw blade weighed 
120 grains, or very nearly double. The directive energy of 
the former, after having suffered great deterioration, and 
when not tempered in the most favourable manner, compared 
with the greatest directive energy of the latter, was as 600 to 
