260 
MR. A. W. RUCKER AKD DR. T. E. THORPE ON A MAONETIC 
The absolute value of the disturbance of the Vertical Force is especially uncertain. 
In the cases of the Declination and Horizontal Force a centre of attraction aftects 
stations on opposite sides with disturbances of opposite signs, to that the true mean 
value for the district can be found, and the true disturbance at each station deduced. 
It is, however, possible that the disturbance of the Vertical Force may always be of 
the same sign. Thus, if tire centres of force are rocks magnetised by induction, the 
north-seeking pole of a magnet would most frequently be attracted downwards in the 
northern hemisphere as the north-seeking poles of the rock magnets would be deeply 
buried in the earth. This hypothesis is that which experieuce has justified in 
Sweden. If this is so, the mean value of the Dip or Vertical Force in a district will 
be greater than its undisturbed value, and negative values do not necessarily indicate 
an upward force. It is therefore safer only to use the disturbances of the Vertical 
Force as a means of indicating relative maxima and minima. 
If, however, centres of repulsion exist, they might be detected by the observation 
that a large negative (upward) disturbance of the Vertical Force was accompanied 
with a sudden reversal of the disturbing Horizontal Force in neighbouring stations. 
If the Horizontal Forces are small, the negative disturbing Vertical Force is more 
probably to be interpreted as indicating a downward attraction of less than average 
magnitude. 
On turning to Plate XL, in which the disturbances of the Vertical Force are 
plotted down, it will be observed that there is, on the whole, an excess of positive 
values in the south-east, and that large negative values are more common in the north 
and west. It is obvious that in Scotland the disturbances are much greater than in 
England, and very large positive values of vertical disturbance occur in the wmstern isles. 
Indeed, if we include the enormous value obtained at Ganna, the average for the whole 
of Scotland is positive. Leaving out, however, this very abnormal station, it can hardly 
be doubted that negative values are more common in the north-west. If this result 
could be trusted as corresponding to physical fact, it might indicate that the country 
as a whole is magnetised in the direction of the magnetic meridian. We cannot, 
however, draw such a conclusion, more especially as it can be shown that the observed 
efiect may probably be a result of our ignorance of the datum lines from which the 
disturbances of the Dips and Vertical Forces ought to be measured. 
We have taken the Dip, as given by direct observation, as one of our fundamental 
elements, and in drawing the terrestrial curves which satisfy the conditions that the 
mean value of the Dip for each district when attributed to the central station is 
accurately reproduced by the formula, we have been compelled to ignore the possi¬ 
bility of an unbalanced downward Vertical Force acting at every station. If we 
suppose that in consequence of this the calculated Dips ought, all over the country, 
to be diminished by a positive quantity, we get for the corresponding decrement in 
Vertical Force 
cZV = H sec"' e dO. 
