362 On the best Kind of Steel and Form 
Polished elliptical-needle, softened in the mid- _Divective force- 
dle by laying it on ared-hot poker .. .. .. .. 610 
Polished rhombus, softened in the middle in the same 
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The needles were now laid aside till the following day, 
when the directive force was again examined. j 
Unpolished rhombus, hard at the ends {Pita io 
Polished elliptical-needle, softened in the middle .. 625 
Polished rhombus, softened in the middle .. .. .. 580 
The polished rhombus was now softened throughout ; and the 
extremities being hardened at a red-heat, the directive force was 
found to be S00). it is scarcely necessary to say, that the needles 
were re-magnetised to saturation previous to each experiment. 
From these experiments I drew the following conclusions : 
That of the steel I émployed, 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 hardened 
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 dif- 
ference 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 parallelogranis 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 clock-spring. ~ 
These needles were made perfectly hard, and magnetised, as was 
always the case, to saturation. 
Experiment 3. 
Needles perfectly hard. eb: 
orce. 
Wide parallelogram 1h ee es en 
Narrow parallelopbram .. .. .. .. 400 
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