Page 335 equipment and instruments 44 



material (paper or cloth), and the instrument placed face down in its case. It should 

 then be wedged by additional padding placed between it and the sides of the case. 



44. NAVIGATION AND POSITION-LOCATION EQUIPMENT 



In this section are described the more important navigation and position-location 

 equipment of survey ships and auxiliary vessels, and their use in hydrographic survey- 

 ing. Ship compasses and compass auxiliaries, the radio direction finder, ship logs, and 

 the taut-wire apparatus are included, but not R.A.R. equipment, which is described 

 in chapter 6. 



441. Magnetic Compass 



All survey ships and most of the auxiliary vessels of the Coast and Geodetic Survey 

 are equipped with at least two magnetic compasses, the standard and the steering com- 

 pass; and several of the larger ships have gyrocompasses (see 442) in addition. 



There are two main types of magnetic compasses, the dry compass and the liquid 

 compass. The Coast and Geodetic Survey uses the United States Navy type of liquid 

 compass on practically all of its vessels, launches, and small boats. 



Complete and detailed descriptions of the magnetic compass may be found in 

 Special Publication No. 96, Instructions for the Compensation of the Magnetic Com- 

 pass; and in the American Practical Navigator (Bowditch). Consequently only a brief 

 reference will be made here to the fundamental principles. 



4411. Ship Compass 



The liquid magnetic compass is a magnet, or several magnets, attached to a com- 

 pass card suspended in a liquid with a low freezing point, usually ethyl alcohol. The 

 card rests on a pivot almost without friction, and the entire assembly is enclosed m a 

 nonmagnetic metal bowl. An expansion chamber is provided to compensate for the 

 expansion and contraction of the liquid caused by changes in temperature. On the 

 inner edge of the bowl is a line called the lubber's Ime, marking the centerline of the ship. 

 The bowl is mounted in gimbals so that it cannot turn in azunuth but will renuxin nearly 

 horizontal regardless of the motion of the vessel, and to reduce, to a large extent, any 

 dynamic effects due to pitchmg and rolling of the vessel. The liquid in the bowl serves 

 the double purpose of damping the oscillations of the card and reducing the weight on 

 the pivot. The standard diameter of the compass card used in a ship compass is 7% 

 inches. All compass cards are graduated in degrees, reading clockwise through 360°. 



A ship compass is usually installed in a compensating binnacle of nonmagnetic 

 metal, located on the centerline of the vessel. 



Movable magnets and soft iron are provided to counteract the effect of the ship's 

 magnetic field. By means of these the deviation of the compass can be reduced to 1° 

 or 2°. The soft-h'on spheres, known as quadrantal spheres, are placed athwartship, one 

 on each side of the bowl and at about the same elevation. Small cylindrical magnets 

 are located in fore-and-aft and athwartship trays beneath the bowl. A Flinders bar 

 is usually located directly forward of the bowl with its axis parallel to the vertical axis 

 of the binnacle. The positions of the magnets are adjustable to provide greater or less 

 effect as needed. (See 4414.) 



