ON COMPASS CORRECTION. 27 



different positions relatively to the earth's magnetising forces as it 

 would if unmagnetised. We are almost without experimental infor- 

 mation upon this subject. Some most valuable experiments are being 

 made, and were communicated to the British Association Meeting at 

 Bristol last September, and they are still being continued by Rowland 

 in the New John Hopkins University at Baltimore, which promises to 

 be the greatest institution for experimental investigation the world has 

 ever seen. He is making exceedingly valuable investigations, the 

 result of which will be most important with respect to the problem of 

 correcting compasses at sea. In the meantime, so far as we know, 

 inductive magnetism takes place quite independently of any permanent 

 or sub-permanent magnetism of the ship. 



The Astronomer Royal shewed that placing steel magnets in proper 

 positions in the neighbourhood of the compass corrects perfectly the 

 effect of the permanent and sub-permanent magnetism of the ship. 

 You will readily understand how that happens. Suppose the ship 

 itself to be a permanent magnet without change of magnetism, then as 

 the ship turns round it carries, so to speak, its magnetic force with it, 

 and if you apply a steel magnet, or set of steel magnets in the neigh- 

 bourhood of the compass whose magnetic force is equal and opposite 

 to the ship, the position beiug found by turning the ship round and 

 adjusting the distance until you find that the magnetic force of the 

 ship is exactly compensated by the steel magnets, then as you turn 

 the ship round into all positions the effect of the permament magnetism 

 of the ship will be annulled, and the compass will point just the same 

 as if the ship were non-magnetic. The ship and the magnets exercise, in 

 fact, a zero magnetic force upon the compass in its actual position. 

 But there remains the effect due to the induction. At first, let us 

 suppose that somehow the effect of the permament magnetism of the 

 ship has been annulled, and that we have nothing but the effect of the 

 magnetism induced according to the different positions of the ship to 

 deal with. Now, it appears, that if the ship is symmetrical, the effect 

 of the induced magnetism on the compass will be zero when the ship's 

 head is north or south, and when it is east or west. You will see that 

 readily. If you imagine the ship to be perfectly symmetrical, and the 

 Compass to be placed amidships, when the ship points due north it is 

 made magnetic by its inductive influence, with its true south pole 



