TERRESTRIAL MAGNETISM, ETC. 315 



perpendicular to the ship's deck, exerted by that 

 part of the ship's magnetism which is induced by 

 the vertical component of the earth's magnetic 

 force. For this object also the proper position of 

 the bar is up and down in the middle of the 

 forward or after side of the binnacle ; but for it the 

 bar should be lowered a little below, or raised a 

 little above, the position in which, without altering 

 the length of the bar, it gives its maximum 

 horizontal force on the compass. When it is not 

 desired to make this contribution to the heeling 

 correction by the Flinders bar, it should be placed 

 with its top about 2 inches above the level of the 

 needles of the compass-card. 



To understand the action of the Flinders bar 

 suppose first the ship to be anywhere in the 

 northern magnetic hemisphere. 1 The vertical 



1 The earth's surface is divided into two parts, called the north- 

 ern and southern magnetic hemispheres, by a line called the magnetic 

 equator, which is the line of no dip. This line is not a great circle 

 like the true equator, but a sinuous line north of the true equator in 

 all east longitude, and from 180 to 173 of west longitude ; and 

 south of the equator in all west longitude less than 173. Its 

 greatest distance on either side of the equator is where it cuts the 

 coast of Brazil in about 17 south latitude. Its greatest distance 



