70 
View of Mr Barlow's Magnctical 
deviation follows a determinate law, so that the amount being 
given in any one case, it may be computed for all others. Let 
us, for example, conceive any two other planes passing through 
the centre of the ball, and each perpendicular to the plane 
QEQ'W, of which let MOSL, M'OSL' represent quadrants; 
then, supposing a compass placed in each of those planes, at 
equal distances from the centre, as at L and L', we shall have 
ML, ML' for arcs of latitudes, and EM, EM' for the longi- 
tudes of position of those points : then the law in question may 
be thus expressed ; 
The tangent of deviation of the compass at L, is to the tan- 
gent of deviation of the compass at L', as the rectangle of the 
sine of 2 LM x cosine EM, to the rectangle of the sine of 2 L'M' 
X cosine EM'. 
E being the east point of the horizon ; consequently the de- 
viation at any one point being given, that at any other, having 
the same distance from the centre, or in the surface of the same 
sphere, may be computed. 
This was an immense step towards the development of the 
mathematical laws of magnetism, which have been for so many 
years buried in obscurity ; but it was not all that was required. 
What is the law observed at different distances ? Subsequent ex- 
periments enabled the author to demonstrate, that, while the po- 
sition as to latitude and longitude was the same, the tangent of 
the deviation was proportional to the cube of the distance. 
Lastly , When the position and distance are the same, what is 
the law of deviation, as it depends upon the magnitude of the 
attracting body P In the pursuit of this inquiry a most unex- 
pected result was obtained. It appeared, in the first place, 
that the tangent of deviation was proportional to the cube of 
the diameter, which might naturally have been expected in solid 
balls ; but this law was found to obtain also in hollow shells : 
in fact, the most solid ball was found to be only equal in power 
to a shell of the same dimension, although, in the latter case, 
the iron should not exceed 5 3 5 th of an inch in thickness ; or, in 
other words, it was thus ascertained that the magnetic power of 
iron bodies resides only on the surface. And, in a subsequent 
series of experiments, it was demonstrated, that these laws have 
place not only in balls and shells, but also in iron bodies of 
