TERRESTRIAL MAGNETISM, ETC, 309 



little or no tidal current, and there anchored, and 

 by aid of a tug or tugs, or by warps and anchors or 

 fixed moorings and buoys laid out in proper 

 positions, turned round all points of the compass 

 and detained on each point on which the error is to 

 be observed, or observed and corrected, long 

 enough to allow the observation to be made and 

 the requisite adjustment performed. 



A very simple method of taking advantage of 

 this process not merely to determine the errors of 

 the compass, but to annul them, which was worked 

 out and published so long ago as 1837 by the 

 Astronomer- Royal, Sir George Airy, has been in 

 practical use, more or less, ever since. It consists 

 in first placing steel magnets in proper positions 

 within a few feet of the compass to correct the error 

 on the north or south, and on the east or west 

 courses, and then applying soft iron to correct a 

 residual error, which is still found after the compass 

 has been corrected on the cardinal courses. This 

 residual error Airy called the quadrantal error, 

 because it has its maximum value in either direc- 

 tion when the ship's head is on one or other of 



