128 
Captain Kater on the best kind of steel 
The surfaces of the cylinders deter nined by very careful 
measurement were, the sheet iron, 28,54, » the chest plate, 
30,77; and the solid cylinder, 28,94 inches. 
Reducing the deviations to the same extent of surface, viz. 
that of the solid cylinder, they become respectively 141, 184, 
and 184 minutes. 
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 certain degree of thickness of the iron (about two 
tenths of an inch) is necessary to the complete developement 
of this effect. 
The following are the principal inferences which may be 
drawn from the experiments I 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 susceptible of the great- 
est directive force. 
That the best mode of tempering a compass needle is, 
first to harden it at a red heat, and then to soften it from the 
middle to about an inch from each extremity, by exposing it 
to a heat sufficient 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 
