38 



HANDBOOK OF MAGNETIC COMPASS 



Therefore, as each adjustment is being made, the steersman should 

 attempt to hold the ship steady on that heading by some means other 

 than the compass being corrected. Adjustments by this method are 

 made as a series of approximations, for example : 



Place the ship on any desired compass course, and correct the 

 compass to read the corresponding magnetic heading. This will prob- 

 ably leave the ship on a course other than the desirable cardinal and 

 intercardinal headings for compass adjustment. For accurate results, 

 the above procedure should be repeated. 



If the compass has no appreciable deviations, the deviations taken 

 on compass headings will closely approximate those taken on magnetic 

 headings. However, as the magnitude of errors increases, there will 

 be a marked difference between the deviations taken on compass head- 

 ings and those taken on magnetic headings. The Napier's Diagram 

 affords a method of converting compass ct)urse to magnetic course, or 

 vice versa, regardless of whether the deviations were taken on mag- 

 netic headings or compass headings. See article 42 for more details 

 concerning its use. ' 



48. Methods of placing ship on magnetic headings. — A ship may 

 be brought on a magnetic heading by reference to a gyroeom'pass. 

 The magnetic variation is applied to true heading to determine the gyro 

 course which must be steered in order to place the ship on the desired 

 magnetic heading. If the gyrocompass has any error^ it must be 

 taken into consideration. It is well to calculate all such problems 

 through true headings^ since short cuts on this procedure frequently 

 lead to errors. Examples of such rolationships are tabulated below : 



The difference between gyro heading and magnetic heading will be 

 constant on all headings as long as the gyrocompass error is constant 

 and the variation does not change. This gyrocompass error may be 

 determined by a comparison of the calculated true azimuth of the sun 

 and the azimuth as observed on a synchronized repeater. 



It is to be remembered that gyrocompasses have certain errors result- 

 ing from latitude and speed changes as well as turning errors, and that 

 these errors are not always constant on all headings. For these rea- 



