Before starting on a long voyage, es­pec­ial­ly with a new ship using the mag­ne­tic compass, it is cust­o­mary to swing the ship through a comp­lete circle to check dev­i­a­tion. To swing ship it is first nec­es­sary to pick out a suit­able ob­ject on land having a known bear­ing to the ship. This object is used as a ref­er­ence point. If at sea ob­ser­va­tions are taken on the sun. The ship is then swung through 360 degrees, stop­ping usually on each 15-degree head­ing, and noting the dev­i­a­tion. A table is made up showing the deviation on each of these headings. An attempt is then made to so adjust or manipulate the compensating magnets to eliminate the error found. The ship must then again be swung through 360 degrees, stopping at headings as before to check the applied compensation.

A Stern-wheeler of early steam-boating days.

Figure 5. Errors of Magnetic Compass Cause Departure From Desired Course.

On some ships it is the custom to check the deviation by the deflector magnet method. The ship in this case is put on a certain heading and a magnet placed to one side of the compass and the deviation noted. The same magnet is then placed at an equal distance to the opposite side and the deviation noted. The difference, if any, between the readings is the deviation on that particular course.

With either meth­od of check­ing for dev­i­a­tion, con­sid­er­able time is used. It is not nec­es­sary to check for dev­i­a­tion or ap­ply any com­pen­sa­tion to the Gyro-Com­pass, as it is not mag­net­ic. In fact the Gy­ro-Com­pass has noth­ing what­ever to do with magnetism.

When at sea the Gyro-Compass af­fords the means of keep­ing to the straight-line, true course. The line A B, Figure 5, shows the straight-line course from the port of New York to the port of Liver­pool. The line A C E B shows, with exag­ger­a­tion, the actual course steered due to com­pass and other er­rors. At the point E the ship’s position was checked by observation of a celestial body. The line E B represents the new course set to bring the ship to her destination. This is an occurrence which sometimes happens not once but often during a voyage.

It is evident that a loss of time is in­volved when the ship leaves her straight line course. The in­her­ent ac­curacy of the Sper­ry Gy­ro-Com­pass en­ables the ship to keep to the straight line course, and also to steer di­rect­ly on true courses.