Page 345 equipment and instruments 4435 



Because of the damping of the oscillations there is a slight lag in the response of 

 a magnetic compass. Precise bearings by peloriis referred to a magnetic compass are 

 only possible when the ship is steady on course and the rudder is amidships. An ob- 

 server should be stationed at the compass, whose duty is not only to read the heading 

 at the instant of observation but to watch the wheel, rudder, ship's head, and the 

 action of the compass for the most propitious moment. 



When referred to the magnetic compass, bearings by pelorus must be corrected 

 for variation and deviation to obtain the true bearmg. Sliips equipped with gyro- 

 compasses have repeaters mounted in the pelorus stands so that all bearings observed 

 are automatically true without corrections for the ship's heading, variation, or deviation. 



4435. Telescopic Alidade 



More accurate bearings can be observed with an azimuth circle or a pelorus 

 equipped with a telescope than are possible with an ordinary sighting vane. This is 

 particularly true in hydrograpliic surveying where bearings are desired on buoys or 

 distant objects, which are almost invisible to the naked eye. 



The telescope should be of a low power with a large field of view which contains 

 a vertical sightmg line. The low power and large field of view are of great assistance 

 in finding an indistinct object. 



A telescopic alidade, which is available for use with the gyrocompass repeaters, 

 resembles an azimuth circle on which is mounted an erecting telescope of about 3 

 power. It is arranged so that the part of the compass card vertically below the line 

 of sight is reflected by prisms into the field of view. A level bubble is also reflected 

 into the field of view, A rotatable prism may be attached for use in taking bearings 

 of celestial objects; with it an object at any altitude can be reflected into the field of 

 view while the telescope is kept level. Since the graduations of the compass card and 

 the level bubble are in the field of view at all times, it is easy to read a bearing even when 

 the ship yaws, rolls, or pitches so much that the telescope can be pointed at an object 

 for only an instant. 



Used with a gyrocompass, bearings are believed to be accurate to a half-degree 

 even under moderately unfavorable conditions, that is, with a moderate sea and only 

 fair visibility ; and still more accurate under favorable conditions. 



4436. Small Theodolite Used as a Pelorus 



A small 4-inch theodolite can be substituted for a pelorus with telescopic alidade 

 with good results. The instrument should be mounted in a suitable location and be 

 well secured. If the instrument tripod is used the legs should be well lashed to a ring- 

 bolt in the deck. The instrument must be oriented so that the 0° and 180° of the 

 lower plate are alined parallel to the lubber's line of the compass (see 4437). 



On a distant object a rapid sequence of observations is made, the ship's head by 

 compass being read at each "mark," The observations should extend over equal time 

 intervals before and after the time of the position. 



The observations should be made at approximately equal intervals, but it is more 

 important that the ship and compass be steady at each observation than that a regular 

 time interval be maintained. 



Each observed bearing is applied to the respective compass heading, and the mean 

 of the series is taken and corrected for the variation and compass deviation to obtain 

 the true bearing, 



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