OF FERRUGINOUS BODIES. 253 
the north pole of which was drawn forcibly to- 
wards it. 
96. Under these circumstances, the needle was 
made to vibrate, and the force was such as to cause 
about thirteen or fourteen vibrations per second. 
The vibrations were so rapid, however, that it 
was difficult to count them with precision ; but 
by taking notice of the appearance of the needle 
on one side of the meridian only, the difficulty of 
counting is much lessened; and by this means I 
was enabled to ascertain that the double vibra- 
tions amounted to more than six per second: 
sometimes there appeared to be nearly seven per 
second: but as no great exactness is required in 
cases of this kind, I have allowed six double or 
twelve single vibrations per second, which is a 
little below the real number which the needle 
performed whilst under the influence of the mag- 
netic force of the iron, when thus in contact with 
the system of bar-magnets. 
97. The magnets being removed, and the nee- 
dle again at liberty, the iron was placed with its 
axis in the magnetic dip, having its upper end 
resting on the brick at the same distance from 
the needle as in the previous experiment. The 
Mm 
