3o6 POPULAR LECTURES AND ADDRESSES. 



wherever she goes and however she turns. This 

 force may be balanced by an equal and opposite 

 force produced by a permanent magnet fixed in 

 a proper position in the neighbourhood of the 

 compass. Again, the transient magnetism induced 

 in the ship's iron by the earth's magnetic force, 

 however the ship may vary in position, whether 

 by turning horizontally or heeling over in one 

 place, or by going to different places on the earth's 

 surface, may be balanced by an equal and opposite 

 force due to magnetism induced in a properly- 

 shaped mass of soft iron fixed in a proper posi- 

 tion in the neighbourhood of the compass. 



Were our temporary supposition of perfect 

 permanence and perfect transientness of the two 

 constituents of the ship's magnetism rigorously 

 correct, it would be quite practicable to thoroughly 

 and accurately perform the whole adjustment. 

 The measurements and calculations required to 

 allow this to be done for any particular ship are 

 only such as, in the process technically called 

 " swinging the ship," and in the subsequent calcu- 

 lation of the numbers A, B, C, D, E, in 



