544 
MR. A. W. RUCKER AND DR. T. E. THORPE OK A MAGKETIC 
The differences between corresponding numbers in the above tables are such as 
might be expected if the directions of the terrestrial isomagnetics, or the distribution 
of secular change, had not been cpuite correctly determined. 
The discrepancies are very small in central districts. In the south-east of England 
and south-west of Ireland the combined errors of Horizontal Force and Dip produce 
considerable differences in the Vertical Forces calculated from the two sets of fio-ures. 
o 
The following numbers are those given for latitude 52°, near the eastern and 
western limits of the area of the survey. 
Longitude. 
H. 
0. 
V. 
Difference. 
2“ E. . . 1 
1886 
1891 
1-8113 
1-8172 
o 
67 
67 
28-9 
31-8 
4-3689 
4-3936 
--0247 
o 
o 
1886 
1891 
1-7446 
1-7410 
68 
68 
56-3 
55-4 
4-5302 
4-5173 
+-01-29 
This shows that when the two surveys are compared, there is in this extreme case, 
as we pass from Harwich to Valentia, a relative change in the datum-plane from 
which the Vertical Force Disturbances are measured, of 0'0376 metric, or 0'003“6 
C.G.S. unit. 
This number is no doubt large, but it must be remembered that the discovery of 
lines of attraction depends upon the comparison of Vertical Forces at neighbouring 
stations, and that, therefore, a gradual slope in the datum-plane will not seriouslv 
affect the results. 
It is also to be noted that if we accept the mean of the results given by the two 
surveys as free from error, the correction which would then have to be applied to our 
1886 survey, would be of the kind that we anticipated. 
In our previous paper (“ 1890 Memoir,” p. 266) we drew attention to the fact 
that positive and negative Vertical Force Disturbances were predominant in the south¬ 
east, and in the north and west respectively. 
We also expressed a doubt as to whether this corresponded with physical facts. 
Now, since the Disturbing Force is the observed less the calculated value of the 
vertical force, an excess of positive disturbances, if unreal, indicates that the calcu¬ 
lated values are too small. Hence, the increase in the calculated Vertical .Forces in 
the south-east of England, and the correspontling decrease in the south-west of 
Ireland, are changes of the kind required to diminish the peculiarities previously 
observed. 
The probability that this kind of error affected our earlier results was also dwelt 
upon in a paper “On the Magnetic Permeability of Pocks and Pegional Magnetic 
Disturbances” (‘Proc. Poyal Soc.,’ vol. 48, p. 532, 1890), published by one of us 
before the observations had been made on which the new terrestrial lines are based. 
