THE CONQUEST OF THE ZONES 



round the world (1772-74), that there is yet another 

 fluctuation of the compass due to the influence of the 

 ship on which it is placed. Considerable quantities 

 of iron were of course used in the construction of wooden 

 ships, and it was made clear that the ship itself comes 

 under the influence of the earth^s magnetism and exerts 

 in turn an appreciable influence on the compass. 

 The fluctuation due to this source is known as devia- 

 tion, in contradistinction to the larger fluctuation akeady 

 referred to as declination. 



Not only is the deviation due to the ship's influence a 

 matter of importance, but it was discovered by Captain 

 Matthew Flinders, in the course of his explorations 

 along the coast of New Holland in the year 1801-02, 

 that the influence of the ship over its compass varies 

 with the direction of the ship's prow. 



Needless to say, the problem of the deviation of the 

 compass due to the influence of the ship is enormously 

 complicated when the ship instead of being constructed 

 chiefly of wood is made of iron or steel. It then be- 

 comes absolutely essential that the influence of vessels 

 shall be reckoned with and so far as possible com- 

 pensated. Such compensation may be effected by the 

 adjustment of bodies of iron, as first suggested by Bar- 

 low, or by the use of permanent magnets, as first at- 

 tempted by England's Astronomer Royal, Professor 

 Airy. At the very best, however, it is never possible 

 totally to overcome the ship's perverting influence, 

 allowance for which must be made if an absolutely 

 accurate conclusion is to be drawn from the record 

 presented by the compass. 



