Any wheel rotating at a high speed about its own axis, and free to
place itself in any plane, is called a _Gyroscope_. The _Gyroscope_ is
the instrument which utilizes the earth’s rotation as a force to direct
the course of ships.
Suppose you were to place such a small wheel supported by its axis upon
a larger wheel which also is revolving. The rotation of the larger
wheel would so influence the smaller wheel that its axis would point
in the same direction as the axis of the larger wheel. Why this is the
case does not concern us here. Let it suffice that the larger wheel
will cause the smaller wheel to behave in this manner. This is in
accordance with a natural law. This law operates as unfailingly as the
law which causes an unsupported body to fall to the ground.
[Illustration: Preparing Master Compass for Test.]
Suppose the larger wheel happens to be the earth, which in reality
is a revolving wheel. Suppose further, the small wheel is a Sperry
Gyro-Compass. In accordance with this natural law just outlined the
smaller wheel, or Gyro-Compass, will point its axis in the same
direction as the axis of the earth, or, in other words, to the true
or geographical North Pole. This explanation of the principle of
gyroscopic motion is necessarily crude. The principle itself has
been established beyond any reasonable doubt. It can be proved by
mathematics to the satisfaction of the most exacting scientist and has
been demonstrated, throughout the navies of the world, to practical
seamen.
The final result is that we have a principle which enables us to
construct an instrument which will place itself in the true geographic
north and south meridian, and that it responds to no influence or
impulse other than the earth’s unvarying rotation.
[Illustration: Galley-slaves drove the Triremes of ancient Rome.]
[Illustration: FIGURE 1.
THE EARTH SURROUNDED BY ROTATING WHEELS AS IT APPEARS TO AN IMAGINARY
OBSERVER LOOKING AT IT FROM THE SIDE.]
[Illustration: FIGURE 2.
THE EARTH SURROUNDED BY ROTATING WHEELS AS IT APPEARS TO AN IMAGINARY
OBSERVER LOOKING DIRECTLY AT THE SOUTH POLE]
[Illustration: A War-ship, “when knighthood was in flower”.]
_How a Compass is Used_
The purpose of a compass is to indicate direction. The relative
position of the North Pole to any point on the earth’s surface is
called North. We figure all direction from this conception. This
geographical North Pole is called the True North. About 800 miles from
this True North Pole is a spot which has a strange magnetic attraction.
The needle of the magnetic compass, if undisturbed by local influences,
points to this spot, and not to the True North Pole. This spot is
called the Magnetic North Pole. This mysterious attractive spot is not
stationary. It moves about from year to year within a wide circle.
Inasmuch as the navigator must refer to True North, he must determine
the angle or variation between True North and Magnetic North as
indicated by his magnetic compass. This determination is made
comparatively easy by using charts which express in degrees the
difference between Magnetic North and True North for any point on the
earth’s surface.
Such a chart is shown in Figure 3. Also on each chart used by a
navigator for a particular locality there is marked a compass rose in
which is recorded the variation for that exact spot as of a certain
date, and in addition the rate at which the variation changes annually,
Figure 4.
Navigation along a coast line where sights can be taken on buoys or
lighthouses is simple, and is termed “piloting.” This, of course, can
be done without the aid of a compass.
[Illustration: Master Compasses Awaiting Shipment.]
Upon getting to open sea the mariner checks his position in a similar
manner, by observing the position of his ship in relation to the
position of the sun, moon or stars. Between observations the position
of a ship is determined by “dead reckoning.” The distance it has
traveled from the last known position is measured by the ship’s log
and the direction is indicated by the compass. Very often for days
at a time, owing to weather conditions, it is impossible to get an
observation or sight on a celestial body. During this run the navigator
is dependent entirely upon the compass. The slightest error in the
compass, due to variation or deviation, in such circumstances will
cause the ship to be miles out of its course, and the actual position
will be far from the calculated position.
[Illustration: Crusaders sailed to Palestine in ships like this.]
[Illustration: Figure 3, Figure 4.]
[Illustration: The Santa Maria carried Columbus to the new world.]
_The Ideal Compass_
If you were to conceive of a compass which would be free from all the
troubles and errors found in most compasses, which would relieve you of
all the worry and care the present compass requires, a compass which
would be accurate and reliable, a compass which would be the Ideal
Compass under all conditions, you would undoubtedly conceive of a
compass that had the following characteristics: