| for a Compass- Needle. 365 
except that the needles became less susceptible of directive force 
from repeated exposure to heat, and that this effect was not occa- 
sioned by a decarbonisation of the steel. The small rhombus of 
saw-blade, perhaps from being the thickest, suffered Jess than 
those made of clock-spring. 
The springs of clocks are made by passing the steel between 
rollers; and it thus undergoes great compression. May not this 
state be favourable to magnetism; and the repeated expansion 
of the steel by heat, destroying this state, have occasioned the 
deterioration I have remarked ? 
The needle which was made of saw-blade having suffered less 
than the others in the preceding experiments, I procured three 
other needles of this material; they were cut out of the same 
plate; the weight of each was 120 grains, and their length four 
inches and a half. One was a parallelogram, 0°46 inch-wide ; 
another a rhombus, as before, 0°87 inch-wide; and the third a 
pierced rhombus, having the middle 1+5 inch, and its sides 0°25 
wide. 
These needles were made without its being found necessary to 
soften the stecl. plate; they consequently were all as nearly as 
possible of the same degree of temper. In this state they were 
magnetised. 
Experiment 11. 
Steel the same as worked. Directive force. 
yParallelorram Ss woe. sia. Vase 
homie se a. Uos. peprcutleon 
Pierced rhombus .. .. «. +. 1085 
Wishing to try whether the needles were magnetised to satu- 
ration, I carefully re-magnetised them. 
Experiment 12. 
Needles re-magnetised. Directive force. 
Parallelogram a xi 4 tee CODE Na 
Rhombus Aa tast Untick Bitte e aereree ed 
Piercedrhombus .. .. «. «+ 1069 
1 now to my surprise found that the directive force, instead of 
increasing, had lessened in each of the needles, and | became 
anxious to discover the cause of so unexpected a phznomenon. 
It has been observed by M. Coulomb, and more fully entered 
into by Biot, that if a needle be magnetised to saturation by 
strong magnets, and afterwards weaker magnets be applied, the 
needle will lose some part of the force it had before acquired. 
Now, if in using the same set of magnets a certain degree of 
force be communicated to a needle, and the magnets he after- 
wards arranged in a manner less favourable for imparting wag- 
netic force, it should seem that this second operation would 
produce the same effect as would fellow the use of magnets of 
les, 
