7. It must not be influenced directly or indirectly by any temperature changes.
8. It must not be influenced by the roll or pitch of the ship.
9. It must not be influenced by any weather conditions.
10. In the event of failure, or error, it should give instant warning.
Comparison of the Magnetic Compass with the Sperry Gyro-Compass
Let us compare the Magnetic Compass with the Sperry Gyro-Compass and determine which more nearly approaches the Ideal Compass.
The Magnetic Compass does not point to True North, it points to Magnetic North, which is about 800 miles from the True North Pole.
The Sperry Gyro-Compass, which is not a Magnetic Compass, and is not affected by a magnetism of any sort, and derives its directive force from the earth’s rotation, points True North. It does not point to the Magnetic North Pole.
[Illustration: A Galleon, the treasure ship of the Spanish Main.]
Every time a ship’s course is laid or changed, or its position noted, the navigator must make and apply calculations to correct the errors caused by variation of the earth’s magnetic fields, and deviation due to local conditions about the ship. Mistakes are frequently made in applying the correction factors by applying them to the wrong side. An error is thus introduced, which in magnitude is twice the correction factor. Instances are reported of ships being 200 miles out of their courses as a result.
The Gyro-Compass requires no corrections since it is undisturbed by variations or any local magnetic conditions. The reading indicated by the Sperry Gyro-Compass is not approximate—it is absolutely and immediately correct. It is not necessary to correct the course every few hours for variation—the navigator is freed from the necessity of making calculations.
After the navigator has made calculations for the deviation errors of the Magnetic Compass, they must be applied by means of manipulating the soft iron globes and compensating magnets. This is an operation requiring such a high degree of skill that only trained men called Compass Adjusters are qualified for the work.
The occasional turning of a thumb nut is the only compensation necessary in the use of a Sperry Gyro-Compass. No tables or curves are required. The ship’s Navigating Officer makes this adjustment with ease.