644 
MR. A. W. RUCKER AND DR. T. E. THORPE ON A MAGNETIC 
if we regard it as a rectangle 5 miles long and 2 wide lying in the plane of the 
compass. The eastern side of the island rises steeply from the shore, and, being 
nearly in the magnetic meridian, would be but feebly magnetized by the Horizontal 
Force. 
The highest point is some distance inland, and the cliifs sink towards the south. 
Hence, we shall certainly exaggerate the effect of this side if we regard it as a vertical 
rectangle 2 miles long and 600 feet high, distant only 200 yards from the place of 
observation, and magnetized by the Horizontal Component of the earth s field 
resolved perpendicular to it. 
As for the part of the island below the sea, we may suppose this plane to be carried 
down to any depth, but, if so, we must treat the more distant western side in the same 
way. This would, of course, be magnetized in the opposite sense, and thus we 
immensely exaggerate the eftect of the submerged part of the nearest face of the 
island, if we replace it by a rectangle ot infinite depth and two miles wide, and if we 
further neglect the more distant side of the island altogether. By this supposition 
we also remove the lower surface of the mass of basalt which would tend to counteract 
the influence of the upper part. 
The observation was made near the north-eastern corner of the island, and the 
magnetization of the northern face would have produced a greater efiect than 
tliat of the more distant southern side, and would have tended to diminish the 
attraction. 
AVe shall, therefore, certainly over-estimate the Horizontal Disturbance due to the 
island if we consider it to be produced — 
(1.) By the upper surface of a plate of basalt 5 miles long and 2 wide, magnetized 
by the earth’s field, the yacht being 200 yards from one of its narrower edges where 
the observation was taken. 
(2.) By a similar vertical plane 200 yards distant, rising 200 yards above the yacht 
and of infinite depth, lying in the geographical meridian. 
(3.) Tf we suppose the yacht to have been in the vertical plane, which bisects both 
the above, instead of being, as was the fact, near one corner of the island. 
(4.) If we neglect all the other faces of the island, which, on the whole, would have 
tended to diminish the Force. 
If A^ and H be the A^ertical and Horizontal Components of the earth s magnetic 
field respectively, 8 the Declination, k the susceptibility of the basalt, 2x the breadth 
of the nearest side of the island, and and i /2 the distances of the nearest and furthest 
sides, the Horizontal Disturbing Force, as calculated on the above assumption, in 
C.G.S. units is 
2k 
V log^ 
y2_ 
y\ 
X+ \ 
.'C ipJ^ -f- 2/3") 
+ H sin 8 1 sin 
d- sin 
-1 
