Astronomical and Nautical Collections. 333 
vanishes, this deviation became 1° by observation; while the cal- 
culation would make it 59’, which is a nearer agreement than could 
have been expected. 
If we suppose two planes perpendicular to the magnetic meridian 
to pass through the centre of the sphere, the one horizontal, the 
other parallel to the dipping needle, the horizontal deviations of 
the compass in these two planes will have, according to the theory, 
a very simple relation to each other: when the right line joining 
the middle of the needle and the centre of the sphere makes the 
same angle in either plane with their common intersection, the 
tangent of the deviation in the horizontal plane will be to the tan- 
gent of the deviation in the other plane, as the cosine of the dip at 
the place of observation to unity. Mr. Barlow’s observations suf- 
ficiently show the truth of this proposition: thus when the middle 
of the needle was eighteen inches from the centre of the sphere, 
the experiment gave in the second plane, at the distance of 45° 
from the line of east and west, a horizontal deviation of 12° 6’, and 
the dip being 70° 30’, it would be inferred that the corresponding 
deviation in the horizontal plane should be 4° 6’, while the obser- 
vation made it only 4°: the difference of 6’ being probably owing 
to the errors of observation. 
The same mode of computation has been applied to the dip, as 
observed by Mr. Barlow under the influence of the sphere of iron; 
and the differences are not greater than are usually found in all 
such comparisons. Thus when the dipping needle was placed in 
the plane of the magnetic meridian, passing through the centre of 
the sphere, and at the distance of twenty inches, making an angle 
of 45° with the direction of the earth’s magnetism, the dip was. 
reduced from 70° 40' to 67° 40’; the calculation gives 67° 46’ 
which differs only 6’ from the result of the experiment, 
In these numerical computations it has been supposed ; first, 
that the action of the earth is the same on the magnetic fluid of the 
sphere magnetized by its influence, and on the fluid belonging to 
the needle employed; secondly, that the action of the fluid of the 
sphere on itself is also equal to the action that it exerts on that of 
the needle. It was natural to make these suppositions in the first 
Vox. XVII | 2A . 
