diurnal variation of the magnetic needle , &c. 345 
greatest horizontal deviations will be to each other inversely 
as the cosines of the dips. In the experiments, although the 
variation and latitude were those at the different places of 
observation, the dip remained the same in all cases : so that 
in the deviations corresponding to those at Port Bowen, no 
increase, arising from an increase in dip, could here take 
place, but on the contrary, a decrease took place from the 
increased distance of the revolving poles. 
This manner of viewing the effects on the horizontal needle, 
as depending on those produced on the dipping needle, will 
serve to explain the nature of the changes which take place 
in the intensity of the horizontal needle. The horizontal 
intensity varying as the cosine of the dip, when the dip is a 
maximum the horizontal intensity will be a minimum, and 
vice versa. Let us now consider what effects will be pro- 
duced on a dipping needle with its centre in the same situa- 
tion as that of the horizontal needle in the preceding experi- 
ments ; the angle between the magnets being 130° and the 
instrument adjusted to correspond to the latitude and variation 
of London. It is manifest that the maximum dip will take 
place nearly when the north pole is on the south magnetic 
meridian ; that is, nearly at the time when the needle passes 
zero from the easterly deviation in the morning towards the 
westerly, or a little before n h A. M. ; and the minimum dip 
will happen nearly when the south pole is on the south mag- 
On the same view of the subject. Lieutenant Foster has recently been enabled to 
connect the changes observed in the horizontal intensity with changes in the dip. 
I must however say, that, should the deviations which are observed when one branch 
of the needle is deteriorated, not be consistent with this law, no one will be more 
ready than myself, to expunge from the philosophical code, a law which is found to 
be inconsistent with observed phaenomena. 
Yy 
MDCCCXXVII. 
