196 
bi-horary values = 10156 and 6 - 997. The amounts for a 
period of 10 hours will therefore be 50 - 78 and 34 - 98 ; but 
the amounts actually observed during the 10 hours, from 
9 p.m. to 7 a.m. are 5079 and 3518, and we are therefore 
entitled to conclude that the disturbing force is almost, if 
not altogether, inoperative during this interval. 
The rate of increase and decrease of the variable force will 
be seen from the following numbers : — 
h . 
20 
+0-21 
+086 
22 
1-06 
2-38 
0 
1-55 
373 
2 
1-45 
4-04 
4 
105 
2-96 
6 
0'44 
1*40 
8 
036 
0-26 
An examination of these numbers shows that the intensity 
of the force increases most rapidly about the time when the 
north pole of the magnetic needle is moving most rapidly to 
the westward. It is at its maximum when the needle is at 
its greatest elongation west ; and its greatest rate of decrease 
occurs at the time when the needle is moving most rapidly 
to the eastward. These coincidences and the fact that the 
force acts in the direction of a perpendicular to the magnetic 
meridian, seem to indicate very clearly that it is directly 
connected with the forces which cause the daily changes of 
magnetic declination. 
In my paper “ On Periodic Changes in the Magnetic Con- 
dition of the Earth, and in the Distribution of Temperature 
on its Surface,” read before the Society, on the 8th of March, 
1864, I suggested that changes in the magnetic condition 
of the earth might produce corresponding changes in the 
directions of the great currents of the atmosphere, and as 
the changes in some of the magnetic elements take place in 
a period corresponding with that of solar spot frequency, it 
occurred to me to examine whether the mean direction of 
the wind at Oxford, in different years, had any relation to 
