28 SECTION- PHYSICS. 



towards the north, and its true north pole towards the south. That 

 magnet being perfectly symmetrical will exercise a force in the direc- 

 tion of the length of the ship, and will either augment or diminish the 

 force that the needle experiences from the earth, but it will not alter 

 the direction of that force. Consequently, the needle will point due 

 north, the magnetism induced in the ship notwithstanding. When 

 the ship's head is east and west, notwithstanding the inductive 

 magnetism of the ship, the perfect symmetry will still cause the needle 

 to point due north. This leads to the simple way of correcting the 

 errors in a symmetrical ship. We know that the error due to inductive 

 magnetism is zero when the ship's head is north or south, or when it is 

 due east or west ; then let the ship be placed north or south, and then 

 let the compass be made to point accurately by fixed magnets, then place 

 the ship east or west, and by another fixed magnet so arranged as not 

 to alter the effect of the first, let the compass be again made to point 

 correctly. Thus, by correcting the compass when the ship is north 

 and south, and again when it is east and west, we are perfectly sure 

 that we have annulled the effect of the permanent and sub-permanent 

 magnetism of the ship, provided always the iron of the ship is sym- 

 metrical. 



It remains to correct the effect of the inductive magnetism of the 

 ship's iron. Now, I must introduce two names. The first is 

 "quadrantal error" that name was introduced by the Astronomer 

 Royal when he brought forward the subject forty years ago and 

 secondly : " semi-circular error," which was the name given by 

 Archibald Smith according to the analogy of that used by the 

 Astronomer Royal. The semi-circular error, is that due to the per- 

 manent and sub-permanent magnetism, whilst the quadrantal is that 

 due to the inductive magnetism of the ship by the horizontal com- 

 ponent of the earth's magnetism. It is called quadrantal, because it 

 has a maximum value in the middle of each of the four quadrants, from 

 north to east, east to south, south to west, and west to north. Thus, 

 the quadrantal error is at its maximum when the ship's head is north- 

 east, and is zero when the ship's head is east. Then it has a maximum 

 in the other direction when the ship's head is south-east, it is zero when 

 the ship's head is south, at the maximum again when the ship's head 

 is south-west, but in the same direction as when it was north-east. 



