322 Mr. Barlow on the temporary magnetic effect 
the effect, and for this reason I shall not trouble you with 
the detail of them. 
It at length occurred to me, that the reason of my failure 
arose from the compound influence under which the needle 
was placed, viz. that of the iron ball and of the earth ; I there- 
fore now neutralized it from the effect of both, by means of 
magnets properly disposed, adjusting it always before the 
rotation to a direction tangential to the ball, so that what- 
ever effect was produced at each point, might at least' become 
decided as to its direction. I now immediately arrived at that 
kind of general law I had been in search of ; for I found 
when things were thus arranged, that whatever might be 
the direction of the axis of rotation, if the motion of the ball 
were made towards the needle, the north end of the latter 
was attracted ; and if from the needle, the north end was 
repelled by the iron, the points immediately in the axis ' 
( when of course the motion of the shell was parallel to the 
needle) being neutral, or those at which the change of 
direction took place ; in other words, if the motion of the 
shell continue the same, and the compass be successively 
placed all round the ball, in that semi-circle (from one axis 
to the other) in which the motion is towards the needle, the 
north end approaches the ball, and in the other semicircle it 
recedes, or the south end approaches ; the points of non 
action being in the two extremities of the axis, and those of 
maximum effect in two opposite points at right angles to the 
axis ; in which two latter the needle, when properly neutra- 
tralized, points directly to the centre of the ball. 
This will be perhaps better understood by reference to 
fig* 3, where S is the shell, ab its axis, and ns, ns, &c. the 
