for a Compass- Needle. | 373 
These last results perfectly coincide with the deductions of 
Mr. Barlow, that the effect of iron on a ship’s compass is as the 
surface, and is wholly independent of the mass; but that a cer-. 
tain degree of thickness of the iron (about two-tentis of an inch) 
is necessary to the complete development of this effect. 
The following are the principal inferences which may be 
drawn from the experiments 1 have detailed : 
That the best material for compass-needles is clock-spring ; 
but care must be taken in forming the needle to expose it as 
seldom as possible to heat, otherwise its capability of receiving 
magnetism will be much diminished. 
_ That the best form for a compass. needle is the pierced rhombus, 
in the proportion of about five inches in length to two inches in 
width, this form being susceptibie of the greatest directive force. 
That the best mode of tempering a compass-needle is, first to 
harden it at red-heat, and then to soften it from the middle to 
about an inch from each extremity, by exposing it to a heat suf- 
ficient to cause the blue colour which arises again to disappear. 
That in the same plate of steel of the size of a few square 
inches only, portions are found varying considerably in their ca- 
pability of receiving magnetism, though not apparently differing 
in any other respect. 
That polishing the needle has no effect on its magnetism. 
That the best mode of communicating magnetism to a needle, 
appears to be by placing it in the magnetic meridian, joining the 
opposite poles of a pair of bar magnets {the magnets being in 
the same line) and laying the magnets so joined, flat upon the 
needle with their poles upon its centre ; then having elevated the 
distant extremities of the magnets, so that they may form an 
angle of about two or three degrees with the needle, they are to 
be drawu from che centre of the needle to the extremities, care- 
fully preserving the same inclination ; and having joined the poles 
of the magnets at a distance from the needle, the operation is 
to be repeated ten or twelve times on each surface. 
That in needles from five to eight inches in length, their weights 
being equal, the directive forces are nearly as the lengths. 
That the directive force does not depend upon extent of sur- 
face, but, in needles of nearly the same length and form, is as 
the mass, 
That the deviation of a compass-needle occasioned by the at- 
traction of soft iron, depends, as Mr. Barlow has advanced, on 
extent of surface, and is wholly independent of the mass, except 
a certain thickness of the iron, amounting to about two-tenths 
of an inch, which is requisite for the complete development of its 
attractive energy. 
LXXY. On 
