and form for a compass needle. 107 
pierced rhombus was the best form for a compass needle. 
I may add, that needles of cast steel were tried, but were found 
so very inferior as to be at once rejected. 
My next object was to determine the effect of polish, and ot 
various modes of hardening and tempering the needles. In 
addition to the former needles, two were made of clock-spring 
of the pierced rhombus form, 5 inches long, 2 inches wide, and 
weighing 66 grains. One of these was first softened, then 
hardened at the ends, and left unpolished; the other, as well as 
the solid elliptical needle of spur steel, was hardened through- 
out, and polished. The needles were then magnetized to 
saturation. 
Experiment 2. 
Directive 
force. 
Unpolished rhombus, hard at the ends 
800 
Polished rhombus, hard throughout 
367 
Polished elliptical needle, hard throughout 
38° 
Polished elliptical needle, softened in the' 
middle by laying it on a red hot poker 
Polished rhombus, softened in the middle' 
[ 
6lO 
in the same manner 
59 0 
The needles were now laid aside till the following day. 
when the directive force was again examined. 
Unpolished rhombus, hard at the ends - 
805 
Polished elliptical needle, softened in the } 
middle j 
625 
Polished rhombus, softened in the middle 
580 
