, 
for a Compass- Needle. 363 
An accident happened to these needles, which rendered them 
unfit for further experiment. It, however, appears from that 
above stated, that the directive force i is nearly as the mass, and 
not as the surface; and that the pierced rhombus is supertor to 
the parallelogram. 
M, Coulomb having found that a needle of the rhombus form 
not pierced, and which he calls une lame taillée en fiéche, was 
susceptible of a greater directive force than a parallelogram, 
I was/desirous of repeating this experiment, as well as of com- 
paring this form with the pierced rhombus. For this purpose 
four needles were made four inches and a half Jong, each weigh- 
ing 63 grains; one was a parallelogram, 0-44 inch wide; an- 
other a rhombus, which I shall call the large rhombus, 0-9 wide; 
the third a pierced rhombus, 1:4 wide in the middle, having 
its sides 0:2 wide: these were made of clock-spring. The fourth 
needle a rhombus, which may be called the small rhombus, 0-4 
wide, was made of that kind of steel which is used for saw-blades, 
and which I believe is shear-steel. This last needle was much 
thicker than the others. 
The steel of which these needles were made had heen exposed 
toa sufficient degree of heat to render it soft enough to be worked, 
and in this state the needles were magnetised. 
Experiment 4. 
Steel soft as worked. Directive 
foree, 
PMMAUCIOP TAN toe pce sw. Ahead fae say, ov 2U 
Small rhombus niu hice ce. h visto RILAMR Toe, 
Large rhombus aie! Ba a Seateta Olea 
Brerced rhombuser e's ea cc Ps aii retsuly doe uieo 
Experiment 5. 
The ends of the needles hardened at an Directive 
obscure red-heat. force. 
Parattelogram'?,, (tise 9 237 02.90 .oroy. es PAB 
Small rhombus HEAT SHVAs, LAST eat) beeeye 
Large rhombus ale 25 1A GED aK tL 
Pierced rhombus Bhh dio: JASIb Badd oh 840 
Experiment 6. 
The ends hardened at a red-heat. Directive 
force. 
Parallelogram /s9 (0 M0 i ee edd dy 9 P42 
Small rhombus DS Mala As ees et eee BO 
Large rhombus POR ade ret si bik oie tts TAD 
Pierced rhombus Ln seh OY MEE TI ESP EAET 
Zz2 Experi- 
