479 
the Density of the Earth. 
hardly more than ought to be produced by the action of gra- 
vity; but the motion then increased, so that, in about a quarter 
or half an hour more, it was found to have moved { or 
division, in the same direction that it ought to have done by 
the action of gravity. On returning the irons back to their 
former position, the arm moved backward, in the same man- 
ner that it before moved forward. 
It must be observed, that the motion of the arm, in these 
experiments, was hardly more than would sometimes take place 
without any apparent cause ; but yet, as in three experiments 
which were made with these rods, the motion was constantly 
of the same kind, though differing in quantity from \ to ij 
division, there seems great reason to think that it was pro- 
duced by the rods. 
As this effect seemed to me to be owing to magnetism, 
though it was not such as I should have expected from that 
cause, I changed the iron rods for copper, and tried them as 
before; the result was, that there still seemed to be some 
effect of the same kind, but more irregular, so that I attributed 
it to some accidental cause, and therefore hung on the leaden 
weights, and proceeded with the experiments. 
It must be observed, that the effect which seemed to be pro- 
duced by moving the iron rods from one near position to the 
other, was, at a medium, not more than one division; whereas 
the effect produced by moving the weight from the midway to 
the near position, was about 15 divisions; so that, if I had con- 
tinued to use the iron rods, the error in the result caused there- 
by, could hardly have exceeded ~ of the whole. 
