412 FIRST REPORT UPON MAGNETIC WORK

defects in the earlier observations, and this likewise explains why
Gunter had not reached the same conclusion as Gellibrand. The latter
appears to have been the first one to publish the fact. Up to this time
no particular pains had been taken to note the date of observation of
the needle's decimation, but now the importance of giving the date
was made apparent.

The phenomenon of the secular variation—called secular for the
reason that it requires many centuries for its fulfillment—is the one
which most intimately concerns the land surveyor, who is obliged to
retrace old lines referred to the compass direction. We shall enter
more fully into this matter in the chapter specially devoted to its
exposition.

The cause of this striking phenomenon constitutes one of the most
refractory enigmas in the whole domain of geophysics. The best
minds have given it their undivided attention. Innumerable and in-
genious theories have been evolved, but the pearl of truth still lies
hidden. Its discovery promises to disclose many another of nature's
secrets. It also seems probable that the two causes, that of the origin
of the earth's magnetism, as well as that of the secular variation, are
so intimately connected that the discovery of the one will include that
of the other. The question proposed by Schuster, as cited above,
should be carefully investigated by experimental physicists and an
exhaustive examination should be made of the resultant effect, by
reason of the earth's rotation, of that part of the earth's magnetism
which is not symmetrical about the rotation axis. No greater nor
grander task has ever confronted the human mind than this of the
investigation of the consequences and interactions which must neces-
sarily result from the motions of our mighty geomagnet. The solu-
tion of this problem will do for geophysics what the accomplishment
of the great task which Laplace set for himself did for celestial physics.

The phenomenon of the secular variation has been nowhere so care-
fully and thoroughly investigated as in this country and, in conse-
quence, in no other country can such precise corrections for the secular
shifting of the magnetic meridians be made, of which we shall give
evidence in another chapter. This has been almost entirely due to
the fact that in no other country has it been necessary to give the