470 FIRST REPORT UPON MAGNETIC WORK

tween 1600 and present date, while the full curve has been drawn
to harmonize with the observations back to the time of Columbus. It
will be seen that there is a marked difference between the two curves
for the date 1500. A similar state of things is revealed at Rome, the
broken curve again representing the law from 1510 to present date,
while the full curve represents the observations which can be
obtained with the aid of the early " compass charts " of the fourteenth
and fifteenth centuries. The departure between the broken curve and
the full one amounts to about 17° for the year 1400! We have
similar indications at other stations of a change in the law of the
secular variation prior to 1600.

The special purpose of the diagram has been to show what relation
the secular variation as obtained at Baltimore bears to the general
phenomenon. Each station bears a somewhat different testimony of
the phenomenon we are studying, and it is only by considering the
collective evidence that we can hope to make headway and be enabled
to say what likely transpired at any one station prior to the records,
or what, is likely to occur at this station in the future. By following
the curves systematically around the globe it is quite possible to con-
struct a composite curve, with the aid of which we can obtain a clearer
conception of this most perplexing phenomenon.

The laws actually governing the secular variation cannot be dis-
covered by simply considering the changes in the magnetic declination
alone, as already explained in another place. We can only hope to
make progress by studying the phenomenon in its entirety, namely,
if we take a magnetized needle and suspend it at its centre of gravity
in such a way that it is free to move in any direction whatsoever, to
the left or to the right, up or down, then we shall find that, under
the influence of the earth's magnetism, the north end of the needle
while still pointing approximately towards the north also points down-
ward and the south end upward. The actual direction assumed by
the needle lies somewhere between a true vertical line and a true
horizontal line, nearer to the former than to the latter in these lati-
tudes. This is the true direction in which the earth's magnetic force
acts. On the compass needle we only have the horizontal component