for a Compass- Needle. ; 361 
M. Coulomb was employed. This instrument, as is well known, 
consists of a fine wire, attached to an index, moveable round a 
circle divided into degrees. To the other end of the wire is 
fixed a cradle, to receive the needle which is the subject of ex- 
periment. The needle being in the magnetic meridian when the 
wire has no torsion, is afterwards forced to deviate from it to a. 
mark distant about 60°, by turning the index, and consequently 
twisting the wire. The number of degrees passed over by the 
index will be as the directive force of the needle. 
The needles which I have described were first made soft, and 
then hardened merely at their ends; they were not polished, 
and were magnetised to saturation. 
Experiment 1. 
Needles soft, and then hardened at the ends. Nakheel sa capes 
needle, force. 
Blister-steel, solid ellipse siephe. ies «ila: oi 909 
————— openellipse .. .. .. 66 520 
Spur-steel, solid ellipse .. .. .. .. 66 540 
— Open ellipses if. iim retain vey 2060 500 
Shear-steel, rhombus .. .. .. «» 49 433 
——— rhombus,with cross pieces of brass 45 435 
By the experiments on magnetism mae by M. Coulomb, it 
appears, that the directive forces of needles of similar form are 
to each other as their masses; the directive force, therefore, of 
a needle of the form of a pierced rhombus of 66 grains wouid be 
expressed, according to the preceding experiments, by 633, 
From many other experiments, which | regret were not re- 
gistered at the time, it appeared that shear-steel was capable 
of receiving the greater magnetic force, and that the pierced 
rhombus was the best form for a compass-needle. 1 may add, 
that needles of cast-steel were tried, but were found so very in- 
ferior as to be at once rejected. 
My next object was to determine the effect of polish, and of 
various modes of hardening and tempering the needles. In 
addition to the former needles, two were made of céock-spring 
of the pierced rhombus form five inches long, two inches wice, 
aud weighing 66 grains. One of these was first softened, then 
hardened at the ends, and left unpolished; the other, as well «s 
the solid elliptical needle of spur-steel, was hardened through- 
out, aud polished. The needles were then magnetised to satu- 
ration. 
Experiment 2. Directive 
force 
Unpolished rhombus, hard at the ends... www, SOD) 
Polished rhombus, hard throughout ..  .. .. .. 3867 
Polished elliptical-needle, hard throughout ‘e-iun@ e 
Vol. 59. No. 289. May 1822. Zz Polished 
