454 FIRST REPORT UPON MAGNETIC WORK

throughout the state. The general custom is to allow a change iii
the direction of the magnetic meridian of about 1° in 20 years, or 3'
per year. Again it is often assumed that everywhere in Maryland
the needle pointed truly north shortly after the year 1800. The
appended tables will give the means of judging as to the amount of
error made by these assumptions. It will readily be seen that over a
large part of Maryland the direction of the needle seemingly never
pointed to the true north during the eighteenth and nineteenth cen-
turies, but bore west throughout this interval.

The tables are based on the researches of the Coast and Geodetic
Survey and represent the very best information at present to be had.
They have been arranged especially for Maryland. The endeavor
was to put the matter in such a shape so that the surveyor could
readily make use of the information the tables contain.

Before passing to our special subject, let us recount briefly the main
fluctuations to which the earth's magnetism is subject. Some of these
fluctuations are periodic in their nature, that is, the fluctuation takes
place during a definite interval of time, at the lapse of which the
needle returns approximately to the position it occupied at the be-
ginning of the interval. Others have no definite period and are
more or less spasmodic in their occurrence.

PERIODIC VARIATIONS.

Of the regularly recurring variations of the magnetic declination,
the most pronounced and striking is the solar-diurnal variation. An
idea as to how the needle varies in the course of the solar day by
reason of this variation can best be obtained from an actual example.
Below we have the mean hourly values of the westerly declination as
obtained at the Washington Magnetic Observatory during the year
1890. The hours refer to the 75th Meridian or Eastern time.