for a Compass- Needle. 57 
then softened them from the middle to within three-quarters of 
an inch of the ends till the blue had disappeared. This was 
done by laying the large parallelogram on a red-hot poker, but 
_ from the thickness of the small parallelogram this heat was found 
insufficient, and that of a lamp was employed. The needles 
were then magnetised as in the last experiment. 
Experiment 35. Directive force. 
Small parallelogram .. «2... 1815 
Large parallelogram .. .. «- 1660 
It occurred to me that the heat employed in tempering the 
large parallelogram might not have been sufficient, it was there- 
fore exposed to the flame of the lamp, but in doing this, 2 small 
piece which weighed 10 grains was broken off from its end. It 
was, however, re-magnetised, and ,the directive force was now 
found to be increased to 1720. 
From these last experiments, it appears that the greatest di- 
rective force was given to the needle when the magnets were in- 
clined to it in an angle not exceeding two or three degrees, and 
that this force is little, if at all, influenced by extent of surface ; 
as I conceive the small difference in favour of the greater surface 
may be attributed to some difference in the quality of the steel, 
or its temper, both of which appear to have very considerable 
influence on the directive force. 
Two needles, the one five, and the other eight inches long, 
were cut out of the same plate of steel; they were of equal 
weight, the short one being of greater width than the other. 
Being magnetised to saturation, their directive forces were as 
follow : 
Experiment 36. Directive force. 
Long parallelogram .. «2 oe 2275 
Short parallelogram .. «. «+ 1198 
They were now hardened at a red-heat, and tempered beyond 
the blue from the middle to within an inch of the extreinities. 
Experiment 37 Directive force. 
Long parallelogram .. .. 42 2277 
Short parallelogram .. =... =. 1865 
If the mean of these two experiments be taken, it will be found, 
as was observed by Coulomb, that the directive force of a needle 
of a greater length than five ‘inches is probably as its length. 
My next object was to repeat the very interesting experiments 
recently published by Mr. Barlow, proving the attraction of iron 
on a ship’s compass to be dependent wholly on extent of sur- 
3A 2 face, 
