When a steel ship is building a sub-permanent magnetism is induced in
its keel, hull, and plates. It causes a compass deviation classed as
“semi-circular.” This deviation must be compensated for.
As a ship moves through the earth’s magnetic fields in its varying
quantities and directions, a temporary and varying magnetism is induced
in the soft iron of the ship. The resultant deviation is classed as
“quadrantal,” and must be compensated for.
The Sperry is not a Magnetic Compass. Hammering, riveting, and moving
through magnetic fields may induce magnetism in the ship, but will have
no effect upon the Sperry Gyro-Compass.
There is no condition of the ship or cargo for which the Gyro-Compass
must be corrected.
[Illustration: An American Clipper, highest type of sailing ships.]
[Illustration]