io8 Captain Kater on the best kind of steel 
The polished rhombus was now softened throughout, and 
the extremities being hardened at a red heat, the directive 
force was found to be 800. It is scarcely necessary to say, 
that the needles were re-magnetized to saturation previous to 
each experiment. 
From these experiments I drew the following conclusions. 
That of the steel I employed, shear steel is the best kind for 
compass needles. 
That the best form for a compass needle is that of a pierced 
rhombus. 
That polish has no influence on the directive force. 
That hardening the needle throughout, considerably dimi- 
nishes its capacity for magnetism. 
That a needle soft in the middle, and its extremities hard- 
ened at a red heat, appears to be susceptible of the greatest 
directive force. 
That the directive force does not depend on the extent of 
surface, but on the mass. 
I might also have inferred, that the needle was capable of a 
greater directive force when wholly softened and hardened 
at the extremities, than when entirely hardened and softened 
in the middle ; but it will appear by subsequent experiments, 
to be detailed, that the difference is probably to be attributed 
to a difference in the degree of heat to which the needle is 
exposed in softening it in the middle. 
My next experiments were made with three needles, two of 
which were rectangular parallelograms of equal length and 
weight, but the one only half the width of the other. The 
third needle was a pierced rhombus ; the whole were made of 
