318 STAFF COMMANDER EVANS AND MR. A. SMITH ON THE MAGNETIC 
does not alter the mean directive force on a compass placed near its centre as regards 
elevation. 
It may be interesting to compare the effect of two solid stanchions placed one on 
each side of the compass with that of two solid spheres, in correcting the quadrantal 
deviation. The effect of the stanchions would be nearly 
whence 
r . 
©. 
D. 
2p 
•500 
30° 0 
•222 
12 50 
4 p 
•125 
7 10 
5j> 
•080 
4 36 
A mast or stanchion placed as we have supposed would generally diminish the heel- 
ing error. 
We may compare the effect on the directive force of a compass on the main deck of 
an iron ship with the effect on a compass in the interior of a spherical shell. 
In some ships the value of X at the main-deck compass is about *75. 
Comparing this value with the expression for the force in the interior of a spherical 
shell, viz., 
F 
we have 
or 
taking * as 24, 
1_2=J- ; 
p 8irx 
1 
£_J_ 
p~ 600 
nearly, or the effect is the same as if the compass were inclosed in a spherical shell of 
an inch thick and 50 feet radius, or half an inch thick and 25 feet radius. 
We may observe that at present one of the great difficulties in deducing numerical 
results as to the effect of rods or plates of iron, arises from our ignorance of the value 
of x for iron used for building or plating ships. We hope to be able on some future 
occasion to be able to communicate to the Royal Society the result of observations 
made for the purpose of determining this value in plates of iron of different kinds. 
