340 
Mr. Christie's theory of the 
intermediate minimum east about 4 h A. M., previous to the 
absolute maximum east about 9 h A. M. Another circum- 
stance, common to all the observations in northern latitudes, 
namely, that the extent of the daily variation increases as the 
sun passes to the north of the equator, and decreases on its 
passage south, so clearly follows from the hypothesis that it 
is scarcely necessary to mention it. 
Observations at Port Bowen. Mr. Foster states that the 
mean time of maximum west deviation, deduced from four 
month’s observations, is n h 49™ A. M. ; and that of the 
maximum east io h oi ra P. M. The corresponding times in 
the table are a little after 10 in the morning and 8 in the 
evening. So that the experiments differ considerably from 
the observations in the absolute times, but agree very closely 
in the interval of time between the two maxima. It is ex- 
tremely probable that the forces brought into action by the 
heat of the sun are not directed, at the same instant of time, 
towards points absolutely fixed, whatever may be the situa- ' 
tion of the needle on the surface of the earth ; and it is easily 
seen that in such an extreme case as this, where the dip of 
the needle is 88°, this may have a considerable influence on 
the changes that take place in its direction : therefore, al- 
though the deviations in their general character may agree 
with the effects produced by the revolution of two poles, yet 
we ought not, perhaps, to expect that they should do so in 
every respect. Whether the times of the maxima arising from 
the rotation of these poles would precede those that would 
actually result from the hypothesis I have advanced, would 
be best determined by experiments on substances having dif- 
ferent conducting powers, and whose contact, like that of the 
