S4 6 
Mr. Christie’s theory of the 
netic meridian, or nearly when the needle passes zero from 
the westerly deviation in the evening towards the east, that 
is before 8 h P. M. : the dip of the needle will be nearly undis- 
turbed about 3 P. M. It is evident also, that, the intensity of the 
north pole being greater than that of the south, the maximum 
dip will exceed the undisturbed dip by a greater quantity 
than this last does the minimum. It appears then, according 
to this theory, that the horizontal intensity ought to decrease 
here until a little before 1 1 o’clock in the morning, when it 
should be a minimum ; that it should increase from this time 
till nearly 8 o’clock in the evening, when it reaches its maxi- 
mum ; and that taking the intensity at 3 o’clock in the after- 
noon as the undisturbed intensity, this should exceed the mini- 
mum by a greater quantity than it is itself exceeded by the 
maximum. 
I am not aware of any extensive series of observations 
having been made near London, on the diurnal changes in 
the intensity of the horizontal needle. Those which I made 
in May and June, 1823,* having been undertaken for the 
particular purposes of pointing out the cause of what had 
been considered an anomalous change in the direction of the 
needle, and of exhibiting and illustrating the particular mode 
of observation, were consequently of a limited nature ; but 
as they are the only ones I can refer to, I shall compare the 
preceding conclusions with the results obtained from them. 
From a mean of these observations it appears, that the hori- 
zontal intensity was the least about io h 3o m A. M., and that 
the maximum happened nearer to 7 h 30“ P. M. than to 9 h 
P. M. : also that, taking the minimum as unity, the excess 
• Philosophical Transactions, 1825. 
