3i8 POPULAR LECTURES AND ADDRESSES. 



modern mail steamers and merchant steamers 

 generally, in which the steering and conning of the 

 ship is done on a bridge forward of the engines, 

 with considerably more than half of the ship behind 

 it. It is also almost certain to be the case for an 

 after steering compass, a few feet in advance of the 

 top of the iron stern-post and rudder-head, in an 

 iron steamer or sailing ship. The second above- 

 mentioned case is what will generally be found for 

 a compass anywhere in the after half of the ship's 

 length, to within two or three yards of the stern- 

 post. Most frequently it is not possible to ascertain 

 which of the two is the actual case until the ship 

 has made a voyage through regions presenting 

 considerable differences of vertical magnetic force. 

 Suppose now the first adjustment to have been 

 made somewhere in the northern magnetic hemi- 

 sphere, and suppose that as the ship goes to places 

 of weaker vertical force, 1 the fore-and-aft correcting 



1 "Vertical force" is a short expression for the vertical com- 

 ponent of the earth's magnetic force. It is reckoned as positive 

 when the direction of its action upon a red pole is downwards, as in 

 the northern hemisphere ; and negative when upwards, as in the 

 southern hemisphere. At the magnetic equator it is zero. The 



