272 THE COMPASS IN MODERN NA VIGATION 



the value of the deviation to which his compass will be subject 

 on any heading of the ship; but in making long cruises and 

 passing into different magnetic latitudes they require unceasing 

 attention, because some of them represent the effects of the in- 

 duction of the earth's magnetic field upon the " soft " iron of the 

 ship, and as the ship sails the ocean she passes through ever- 

 varying fields of terrestrial magnetism. Her own magnetism is 

 also undergoing continual, though small, changes due to the 

 wrenching and straining of the ship by the action of the sea. 

 Yet b}^ examining thoroughly into the harmonic coefficients and 

 by considering the known values of the elements of the earth's 

 magnetism, a careful navigator may predict a table of deviations 

 for his ship and compass in any part of the world. 



He will then understand and be prepared for such changes in 

 the ship's magnetism as arise from the heeling of the ship, from 

 change in geographical position, and from alteration in the course 

 after the ship has remained for a long time on one heading, and 

 he may navigate his vessel with the confidence and security that 

 he would have in a wooden ship, for he can at any time correct 

 the course steered by the compass so that the magnetic course 

 actually made good may be laid down upon the chart or used in 

 the calculation of the ship's reckoning, he can correct bearings 

 of the land by the amount of deviation due to the direction of 

 the ship's head at the time they were taken, and if he wishes to 

 shape a course for a port, having found by calculation or from 

 the chart the correct magnetic course to be made good, he can so 

 appl}'' the deviation as to obtain the compass course to be steered. 



In many modern ships the deviations are largely reduced by 

 introducing magnets into positions near the compass to compen- 

 sate for the effects of the ship's magnetism. The analysis of the 

 table of deviations shows that the polar forces acting in the ship 

 may be represented by imaginary magnets, and it is, therefore, 

 certain from well known laws of magnetic action that the effects 

 of these disturbing forces may be neutralized by introducing real 

 magnets whose forces have the same magnitudes but act in the 

 opposite directions. 



The proceedings of the British Association at Toronto were 



admirably reported by the local press, the daily reports of the 



(rfoSe, together with a finely illustrated supplement, aggregating 



nearly 150 columns, or the equivalent of an octavo volume of 



, 550 pages of long primer. 



