THE GUIDING-NEEDLE ON AN IRON SHIP. 303 



would point to the north, at every station. But an iron ship is 

 there : so at station 1 we find the north end of the needle repelled 

 from the vessel ; the same occurs at stations 3, 4, 16, and many 

 others from bow to keel around the forward body of the ship. 

 Now, only north magnetism can produce this kind of deflection : 

 it varies in degree at each station, and where greatest there is its 

 pole. Again : at stations 7, 8, 9, etc., we find the needle's north end 

 attracted toward the ship ; hence we have discovered the body of 



/2 



17 



2,9 



ffiO 



■35 



PiQ. 13.— Plan op a Dry-Dock with a Ship in it. 



Fia. 14.— Vertical Section of Shlp and Dock through the Line D D ov Fig. 13. 



south magnetism, for that alone can produce this phenomenon, 

 and as with the other, so here, we locate its pole where the deflec- 

 tion is greatest. Finally, at stations 5, 6, etc., in an irregular path 

 from bottom to rail we see that the needle points everywhere to 

 the north : this is the neutral line, A sketch of each side of the 

 ship is drawn on paper, and the degree of deflection at every 

 station is plotted by means of the measurements from the line 

 A, B and from the ship's side. 



It has thus been shown that it is a huge magnet, the ship. 



