MARYLAND GEOLOGICAL SURVEY 479

With the aid of the preceding data the final table (No. XII) has
been prepared, giving the values of the magnetic declination in
degrees and minutes, for every five years between 1700 and 1900 at
each of the county-seats. This table was obtained thus: First, an
auxiliary table was constructed giving the secular changes in the decli-
nation for every five years during the interval 1700 to 1900 at eight
different localities well distributed over the state. These eight locali-
ties were obtained by suitable combinations of the C. and G. S. secular
variation stations, as given in the preceding table. They are as
follows:

[See page image for equations]

The formulae explain the various combinations, the letters standing¹
for the stations used. For example, in I, A stands for Athens, M for
Marietta, Ch for Chambersburg, and Hu for Huntingdon. First,
Chambersburg and Huntingdon were combined, giving the former
double weight; next the mean of Athens and Marietta was taken;
and finally, the mean of the two means was formed. In order to-
check the results obtained for each locality they were represented
graphically and compared. In this way slight improvements were
obtained.

The auxiliary table is exhibited below in a somewhat condensed
form. If D₁₉₀₀ and D_t represent, respectively, the values of the mag-
netic declination on January 1st, 1900, and at some time t between
1700 and 1900, and e is the change in the declination between t and
1900, we have

D_t = D₁₉₀₀ + c.

West declination is reckoned as positive and east declination as nega-
tive. The quantities c are given in the auxiliary table with the proper
sign. Suppose the value of the declination at Hagerstown on Janu-