ON THE POLARITY OF THE DIAMAGNETIC FORCE. 
253 
XIX. 
Cylinder of Caermarthen Slate, 
length 4 inches, 
diameter 0‘7. 
Current direct. 
Position 1. 664 
Position 2. 690 
Position 3. 720 
Current reversed. 
300 
235 
185 
The deflections in this case are also indicative of magnetic polarity. 
These two cylinders were so taken from the rock that the axis of each lay in 
the plane of cleavage. The following experiments, made with a cylinder of the same 
size, show the capability of a rock of this structure to be magnetized across the 
planes of cleavage. 
XX. 
Cylinder of Slate : axis of cylinder perpendicular to cleavage. 
Current direct. 
Position 1. 655 
Position 2. 678 
Position 3. 695 
Current reversed. 
240 
205 
192 
Chloride of iron was next examined : the substance, in powder, was enclosed in a 
single glass tube, which was attached to the string passing round the wheels WW' of 
the instrument. 
XXI. 
Cylinder of Chloride of Iron. 
length 3'8 inches, 
diameter 0*5. 
Current direct. 
Position 1. 185 
Position 2. 
Position 3. 990 
Current reversed. 
990 
230 
185 
The deflection here indicates ordinary magnetic polarity. The action was very 
powerful. When swiftly moving in any direction, a change in the position of the 
cylinder instantly checked the magnet in its course, brought it to rest, or drove it 
forcibly in the opposite direction. The numbers 185 and 990 mark indeed the utmost 
limit between which it was possible for the magnet to move ; here it rested against 
the helices. 
Two glass tubes were filled with red oxide of iron and examined. The action of 
the poles of these cylinders upon the magnets was so strong, as to efface, by the velo- 
city imparted to the magnets, all distinct impression of the numbers on the scale. 
2 L 
MDCCCLVI. 
