I 
induced in iron bodies by rotation. 321 
seen above the shell, is a small stand and sliding wire, car- 
rying a common lamp glass, in which a very small dipping 
needle was suspended by silk ; and when the lamp glass 
was out of the ring, the latter served for setting the hori- 
zontal needle on, so as to bring it over any required point 
of the shell. It should be observed that the pedestal was 
moveable, and might therefore be placed on either side of 
the machine. The stand and upright figure 2, is one of two 
large magnets ultimately employed for neutralizing the 
needle. 
The machine being thus prepared, I screwed it down ; 
first with its axis in the magnetic meridian, and then placed 
the compass successively at -the several points on the table 
all round, and registered the deviation produced at each, 
with the motion of the shell direct and reversed. I then 
removed it, and placed the axis east and west, and again ' 
registered in the same manner ; but the results were very 
irregular with respect to quantity. Although I obtained some 
uniformity regarding direction only, viz. in both cases I found 
four points of change at about 30° from each extremity of the 
axis, or four points of non action. For example, when the 
axis was in the meridian from N 30° E to N 30° W, the motion 
of the needle arising from the rotation was made to the right. 
From N 30® W to S 30° W to the left. From S 30° W to S 30° E 
to the right. From S 30° E to N 30® E to the left ; the direc- 
tion of motion in the shell being the same ; with the direc- 
tion of motion reversed, the deviation was reversed also. 
While at these four points themselves, the needle had no 
motion. I tried also a variety of other positions, but I could 
obtain no such results as to lead to a concise expression of 
