MARYLAND GEOLOGICAL SURVEY 467

North end of compass needle
Station. veered between 1800 and 1890.

London.......................... 6° 30' to the east.

Paris............................ 6 58 "

Rome............................. 6 29 "

Manila........................... 0 37

San Francisco.................... 3 02 " "

Baltimore......................... 4 14 " west.

Rio de Janeiro.................. 11 18 " "

Ascension Island................. 9 00 " "

St. Helena ".................. 6 42 "

Cape Town....................... 3 48 " "

The compass needle accordingly while swinging to the eastward at
London between 1800 and the present time was swinging in the oppo-
site direction, westward, at Baltimore during the same interval of time,
the amount of swing not being the same at the two stations.

Another striking fact disclosed by looking over the figures for any
one station, for example, Baltimore, is that at the same station the
change per year, as frequently assumed by the surveyor, is not a
constant quantity. The annual change for this particular station may
vary all the way from zero to four minutes. At the times of maxi-
mum or minimum values of the declination the annual change is
practically zero for about five years on either side of these epochs.
The annual change then begins to increase until about midway be-
tween the epochs of maximum and minimum values, for example,
about 1730 or about 1870, when it reaches its maximum value of
about four minutes; it then diminishes again.

The secular motion of the compass needle may be likened to the
swinging of a pendulum. At the extreme positions of the pendulum,
on either side of the position it would occupy if at rest, the velocity
with which the bob moves in its orbital path vanishes. As the pen-
dulum moves towards its mean position, from the right let us say, it
does so at a constantly accelerating pace until it reaches the mean
position midway between the two extreme positions. Here the
velocity is a maximum, and as the pendulum swings past the mean
position it begins to slacken its pace until reaching the extreme posi-
tion on the left, when the velocity of motion again vanishes. As
stated, at no station has as yet a complete swing, for example, from