and form for a compass needle. 109 
clock spring. These needles were made perfectly hard, and 
magnetized, as was always the case, to saturation. 
Experiment g. 
Needles perfectly hard. 
Directive 
force. 
Wide parallelogram 
490 
Narrow parallelogram 
490 
Pierced rhombus - 
532 
An accident happened to these needles, which rendered 
them unfit for farther experiment. It however appears from 
that above stated, that the directive force is nearly as the mass, 
and not as the surface ; and that the pierced rhombus is supe- 
rior to the parallelogram. 
M. Coulomb having found that a needle of the rhombus 
form not pierced, and which he calls “ une lame taillee en 
fleche”, was susceptible of a greater directive force than a 
parallelogram, I was desirous of repeating this experiment, 
as well as of comparing this form with the pierced rhombus. 
For this purpose four needles were made four inches and a 
half long, each weighing 6g grains ; one was a parallelogram, 
0,44 inch wide ; another a rhombus, which I shall call the 
large rhombus, 0,8 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, w 7 as made of that kind of 
steel which is used for saw blades, and w r hich I believe is shear 
steel. This last needle was much thicker than the others. 
