382 Mr. Christie on the diurnal deviations of the 
At different times on the 14th of May, after observing the 
directions of the needle No. I, I found its time of vibration 
at the south point of equilibrium. This was not done with 
the expectation of being able to draw any very decided con- 
clusions from these observations, since the suspension of the 
needle was not sufficiently delicate to admit of its making 
many vibrations under a diminished force, nor had I then 
more accurate means of ascertaining the time than a good 
watch with a seconds’ hand; but to see how far the change 
in the ratio of the intensities of the terrestrial forces and those 
of the magnet would be sensible. The times required for 
eight vibrations of the needle at different hours in the day 
were as follow : 
At 
Vibrations. 
6 h A. M. - 
- 8 - 
- in 50 
10 - - - 
- 8 - 
- - 52 
Noon - 
- 8 - 
- - 52 
.1 P.M. - 
- 8 - 
- - 52 
4 - - - 
- 8 - 
- - 50.4 
9 - - ~ 
- 8 - 
- - 50 
The differences here are such, that, even with the means I 
possessed, they could not arise from errors in the observations, 
and they point distinctly to a diminution, during the middle 
of the day, of the forces tending towards the point about 
which the vibrations were made ; that is, to an increase in the 
ratio of the terrestrial forces to those of the magnet. With 
a very delicate suspension, such as Hansteen made use of, 
and very accurate means of measuring the time, I consider 
that the times of vibration of a needle so adjusted would give 
a ready and accurate measure of the ratio of the terrestrial 
forces to those of the magnet, and, if taken under precisely 
