4412 HYDROGRAPHIC MANUAL PaGE 336 



For illumination, the ship compass has a translucent compass card and bowl 

 and the card is illuminated from below. A hood for the binnacle is provided to protect 

 the compass when not in use; the hood is provided with kerosene lamps for emer- 

 gency use. A magnifying glass is provided which can be placed where it will give the 

 helmsman a magnified view of that part of the card adjacent to the lubber's line and 

 permit more accurate steering. 



4412. Use of Compass in Hydrography 



Because of its location, the standard compass is less influenced by the vessel's 

 magnetic field than the steering compass, and consequently its deviation remains more 

 constant and can usually be reduced by compensation to smaller residuals. For this 

 reason all long courses in navigation or hydrography should be set by the standard 

 compass, the relative courses on the steering compass, by which the helmsman steers 

 the vessel, being determined by simultaneous comparisons. The headings of the 

 standard and steering compasses should be compared hourly when underway. 



All bearings and deadreckoning depend on the compass heading for azimuth. In 

 offshore hydrography the accuracy of dead reckoning between fixed positions depends 

 directly on the course and the log distances, supplemented at times by astronomic sights. 



The standard compass is usually installed on the flying bridge, or the deck directly 

 above the pilothouse; and the steering or wheelhouse compass is in the pilothouse just 

 forward of the steering station. Each is on, or parallel, to the centerline of the vessel and 

 as far from magnetic metals as possible, and where the magnetic influences of the sur- 

 rounding metals will be as evenly distributed as practicable. 



Portable objects consisting of, or containing, magnetic metals shall not be allowed 

 to remain in the vicinity of the compasses, for the magnetic field may be disturbed 

 with a resulting change in the deviation. For the same reason, a vessel should not lie 

 alongside a dock contahiing a large quantity of magnetic metal. 



A compass shall be stowed face down when not in use, to preclude wear on its pivot. 



For hydrography controlled by visual fixes or R.A.R., the vessel is steered by ref- 

 erence to the compass in an attempt to follow a chosen sounding line. The position of 

 the vessel with reference to the line is determined by successive sextant fixes or bomb 

 positions, and frequent alterations in course are made to bring the vessel back on the 

 sounding line. 



4413. Variation and Deviation 



For all practicable purposes the needle of the magnetic compass would point to 

 magnetic north if influenced only by the earth's magnetic field. However, this condition 

 is never attained on shipboard, except in specially constructed nonmagnetic vessels, 

 as the ship itself sets up a magnetic field which causes the needle of the compass to 

 deviate from magnetic north. 



The variation of the compass, or magnetic decli7iation, is the angle between true 

 north and magnetic north, the latter being the direction assumed by a magnetic needle 

 when acted on solely by the earth's magnetic field, that is, exclusive of any instrumental 

 error and any effect of the ship's magnetism. The angle between magnetic north and 

 the north indication of a ship compass on any particular heading of the ship is the 

 compass deviation for that heading, and includes the instrumental error of the compass 

 as well as the effect of the ship's magnetism. 



