472 FIRST REPORT UPON MAGNETIC WORK

the aid of which it has been possible to give in a convenient tabular
form, for various points in Maryland, the figures enabling the surveyor
to obtain the difference in the direction of the magnetic meridian
between any two dates for the eighteenth and nineteenth centuries.

[See page image for table]

FIG. 5. —The curves described at London and Baltimore, in consequence of the secular variation of the
earth's magnetism, by the north end of a freely suspended magnetic needle 40 inches (101. 6 cm. ) long.
The curves were constructed supposing the observer to be standing at the centre of suspension of the
needle and looking toward the north end. It will be noticed that as far as they can at present be drawn,
they both proceed in the same direction as the bands of a clock.

The observations, next page, can be represented very closely by the
following formula deduced at the Coast and Geodetic Survey office:

D= + 3°. 38 + 2°. 72 sin (l°. 4m — 22°. 3), (1)

where D stands for magnetic declination, plus when west, and m is the
time interval in years and decimals of between date of observation, t,
and the year 1850. We have m = t — 1850. The coefficient of m,