and form jor a compass needle. 105 
being 66 grains. They were of the form of a long ellipse, 
in length 5 inches, and in width half an inch. One of each 
kind was pierced, as in the figure below, the weight being 
made up by additional thickness. This needle, therefore, had 
much less extent of surface than the solid ellipse. 
Recollecting to have had in my possession, many years 
since, a compass of extraordinary power, the needle of which 
was composed of pieces of steel wire put together in the shape 
of a rhombus, I caused two needles to be made of this form of 
a piece of clock-spring, which I understand is of that kind of 
steel which is called shear steel. They were shaped as below; 
in one the cross piece was of brass, and in the other formed 
of part of the clock spring. These needles were, by mistake, 
made to weigh only 45 grains. 
In ascertaining the directive force, the balance of torsion of 
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 sub- 
P 
MDCCCXXI. 
