Page 339 equipment and instruments 4416 



the aurora borealis, and are world-wide in effect. The effect of a magnetic storm on a 

 ship swing in low latitudes is practically negligible as the most severe storm will not 

 affect the compass more than }i°, but in high latitudes the effect may be as much as 



6° to 10°. 



4416. Boat Compass 



The liquid type of boat compass has been adopted as standard by the Coast and 

 Geodetic Survey. In general it differs from the ship compass only in size and port- 

 ability. Various sizes, generally of 4, 5, or 6% inches in diameter, are used, the larger 

 in auxiliary vessels and the smaller in pulling boats and ship launches. The cards are 

 graduated in degrees clockwise through 360°, with no more than the cardinal and inter- 

 cardinal points shown. A binnacle equipped with lights operated from the launch 

 batteries or an oil lamp is provided for use at night. The 5-inch compass has been 

 found most satisfactory for general use, especially in launch hydrography; a smaller 

 compass is too sluggish and is generally unsatisfactory in a choppy sea and a larger 

 compass is too cumbersome. 



A boat compass can be mounted in a noncompensating binnacle near the steering 

 station, or mounted on its box as a stand. It should be installed on the centerline of 

 the boat. Tliis line should be determined and the position selected for the compass 

 station marked so that the compass can always be replaced in the same position. 

 Other points to be guarded against in its installation are similar to those for the ship 

 magnetic compass (see 4411). 



442. Gyrocompass 



All modem survey ships of the Bureau and several of the older ships are equipped 

 with Sperry gyrocompass systems, in addition to magnetic compasses. On a gyro- 

 compass all directions are true, as the effect of magnetism is eliminated. There is no 

 variation or deviation to be taken into account. 



To ensure correct functioning of the gyrocompass it should be compared with one 

 of the magnetic compasses at 15-minute intervals. At least once each watch it should 

 be checked by an observation for azimuth on a celestial body. The result of this obser- 

 vation and one comparison with the magnetic compass each watch should be entered in 

 a compass log book. Should any comparison indicate that the gyrocompass is not 

 functioning properly, hand steering with reference to the magnetic compass shall be 

 resorted to immediately and continued until the fault in the gyrocompass has been 

 corrected. 



In some installations it is not possible to set the steering and bearmg repeaters 

 closer than /4°; in such case the relationship of the repeaters and the master compass for 

 each trip should be recorded and the bearing repeater used for each bearmg should 

 be noted. 



When the comparison between the gyrocompass and the magnetic compass differs 

 consistently from normal by 1° or more for a short time, a local magnetic attraction 

 may be indicated, and the possibility should be noted in the log book for future inves- 

 tigation. 



4421. Theory of the Gyroscope 



A gyroscope is a well-balanced wheel, revolving at a high rate of speed, with freedom 

 of motion about three rectangular axes. The phenomenoji of the gyroscope is that its. 

 axis of rotation points in a fixed direction in space, if no outside force is exerted on it. 



