Written to enable the owner or skipper of a small vessel to ascertain the deviation of his compass and correct it. Principal causes of deviation are described. Methods shown are simple and have been used by the author in correcting the compasses of small craft up to 70 tons. Correction of a compass in a large steel vessel is a more complicated business, and needs the services of a professional Compass Adjuster. Contents: Description of a Liquid Compass; Removing a Bubble; Taking a Bearing; Pelorus; Earth's Magnetism, Lines of Force, etc.; Permanent Magnetism; How Deviation is Caused, etc.; Induced Magnetism; Quadrantal Correction Heeling Error, etc.; Precautions before Swinging Ship; Swinging Ship; Finding Deviation Under Weigh, etc.; Correcting the Compass Finding Positions by Two Horizontal Angles, etc.; Deviation by the Sun; Amplitude.