536 
MR. A. W. RUCKER AND DR T. E. THORPE OK A MAONETIC 
We there gave reasons for believing that the secular change of Declination 
(pp. 89 -91) and of Dip (pp. 83 and 87) has of late been less rapid, and as our 
previous table was based upon a comparison of our observations with those of Sir 
F. Evans, in 1872, and of earlier observers, it may w^ell be that the numbers were a 
little too large. 
Data as to variations of the Horizontal Force are more scanty, but there is reason 
to believe that, in the case of this element, the rate of secular change is increasiim in 
Western Europe (“ Memoir 1890,” p. 92). 
Reduction of ObservatiOx\s made during the 1886 Survey to the Epoch 
OF THE New Survey. 
Having determined the rate of secular change during the interval 1886-91, we 
reduced all the observations of which an account was given in our earlier Memoir to 
the epoch January 1, 1891. 
These reduced observations, together with the Vertical Forces calculated from 
them, are entered, together with the additional results more recently obtained in 
Table XXIV. (p. 556). 
In tills table the two surveys are treated as one. In each of the three main 
divisions, viz., Scotland, England and Wales, and Ireland, the stations are arranged 
alphabetically. 
They are numbered consecutively, but in the case of places at which observations 
were made in 1884-88 the numbers attached to them in our “ 1890 Memoir” are 
also given. 
The values of the Declination, Dip, Horizontal and Vertical Forces are entered in 
red figures on Maps 5-9, and the true isomagnetlc lines are shown in blue. They 
ore drawn, not for equal differences in the values of the elements, but for such values 
as are most convenient for exhibiting the general form and irregularities in the 
lines. 
In our previous paper we discussed at length the methods of drawing from these 
lines conclusions as to the nature of the disturbing forces, our object being to show 
that the direct results of observation led to the same conclusions as those which can 
be more easily drawn from the Disturbing Forces as calculated by our methods. M e 
do not think it necessary to enlarge here on a proposition which has been sufficiently 
proved, and has received a strong confirmation from the agreement between the two 
surveys which has been already demonstrated in this paper. 
Some facts, however, connected with the true isomagnetics deserve attention, but 
we reserve comment on them till the sections in v'hich we deal with the disturbances 
in detail. 
It is only necessary to remind the reader (1) that the curves are drawn freehand, 
and that we do not aim at Indicatino’ more than that the line at which an element 
L7 
