186 ON MAGNETISM. 



82. Examination of the heeling -disturbance, and 

 remarks on the possibility of correcting it. 



The quotient of the deviating force by the terrestrial 

 directive force, on which the needle's deviation will 



Y 

 depend, will be (remarking that ^y= tan dip) 



sin h x \N. cos 2 A -f ( R tan dip + TT ) cos A [ . 



No simple rule can be given for the position of the 

 ship's head which will make the bracket vanish : cos A 

 will be determined by a quadratic equation. 



The first term has for factor N. Now in examining 

 Articles 68 and 69, it will be seen that N is that effect of 

 induction which puts on the appearance of a constant 

 magnetic force parallel to the ship's keel. The correction 

 of N by a magnet is of no avail in reference to the 

 formation of the first term in the last article. But 

 correction of JVby a mass of iron subject to the same 

 induction as the rest would destroy the term in the last 

 Article. In the ordinary place of the steering-compass 

 in a merchant-ship, it may happen that this term is 

 negative and large, principally as affected by the magnet- 

 ism of the sternpost : and the treatment of the heeling 

 error is very unmanageable. There appears to be no way 

 of determining the value of the bracket in different 

 azimuths, except by inclining the ship in different 

 azimuths. Here we see a great advantage in the use of 

 Mr. Rundell's vertical bar in front of the compass. This, 



