178 Lieut. Foster’s comparison of the diurnal changes 
in the following Table, was to ascertain, by several series 
of vibrations made with the same needle, mounted alternately 
as a dipping needle, and as a horizontal one, whether or no a 
corresponding variation of intensity would manifest itself in 
these two positions respectively ; as ought to be the case, if 
the diurnal changes of intensity in the horizontal needle 
proceeded from a general change of intensity in the terres- 
trial magnetic power. But on the other hand, if the force 
indicated by the dipping needle should be found to remain 
constant, then it would be equally clear, that the variations 
of intensity in the horizontal needle proceeded from an 
actual change of dip only. 
As this question is of considerable importance in the theory 
of terrestrial magnetism, I regret that I had not an opportu- 
nity of making a more extended series of experiments of 
this kind ; but, as far as they go, they certainly appear to 
indicate, that the alterations of intensity in the horizontal 
needle, are due rather to a daily change in the amount of the 
dip, than to any variation in the general intensity of the earth’s 
magnetic force ; although some change in this also is observ- 
able by the vibrations of the dipping needle This explana- 
tion of the cause of the change of horizontal intensity, it may be 
remarked, is consistent with the observations made in Europe, 
which likewise show an alteration of intensity in the hori- 
zontal needle during the day, but in a much less degree than at 
Port Bowen. Now, if the variation in question really proceed 
from a change of dip, to the amount of 3, 4, or 5 minutes of 
a degree, the change of intensity in the horizontal needle 
will be less and less obvious, as the dip decreases ; but if it 
proceed from a change in the actual intensity of the earth’s 
