horizontal needle when under the influence of magnets. 381 
forces, would cause the deviations of the easterly and westerly 
points of equilibrium to be always both plus or both minus , that 
is, that the one would always approach the north whilst the 
other receded from it ; that the effect of a change of intensity 
alone of these forces was, to make the deviation of the one 
point plus, whilst that of the other was minus, or to make them 
approach and recede from the north together; and that if a 
change took place at the same time both in the intensity and 
direction, the westerly and easterly points might both approach 
and both recede from the north, or the one approach and the 
other recede, according as the one cause or the other produced 
the greater effect ; but that their both approaching or both re- 
ceding from the north, at any time, must arise from a change 
of intensity. Comparing then the whole of the preceding 
observations with these effects, it is evident that the changes 
which take place cannot be explained by a change in the di- 
rections alone of the terrestrial forces, but that their charac- 
ters agree, as nearly as we can possibly expect, with the 
effects that would take place from an increase of intensity at 
the time that the direction deviated towards the west : we are 
therefore led to infer, that such an increase of intensity must 
take place in the terrestrial force during the time of the 
westerly deviation. The change of intensity during the day, 
has been already ascertained by the observations of Hansteen 
on the vibrations of a needle very delicately suspended, but 
in the present state of our information respecting the mag- 
netical phaenomena of the earth, the series of observations 
which I now present will not, I trust, be considered without 
interest, although they should, at first sight, appear only to 
confirm results obtained from previous observations, 
