f/. ( 30 ) 



Observations on the Deviation of the Compass ; with Examples 

 of its fatal iiifluence in some melancholy and dreadful ship- 

 xvrecks*. By the Rev. William Scoresby, F.R.S., &c. 

 (Communicated by the Author.) 



A HE deviation of tJie compass on shipboard, is that error or 

 anomaly in the needle, from the correct magnetic meridian, pro- 

 duced by the magnetic condition of the vessel. It is but a mo- 

 dern discovery, and, until within a very few years, did not ob- 

 tain much consideration ; and even now, is very far from having 

 obtained that general attention to which its great importance in 

 practical navigation so abundantly entitles it. A few personal 

 observations, and well ascertained facts, will be sufficient to prove 

 that a correct knowledge of the deviation must greatly contri- 

 bute to the safety of persons traversing the ocean ; and that ig- 

 norance of it must expose all persons engaged in commercial 

 transactions by sea to a fearful risk (a risk by no means general- 1 

 ly appreciated or accredited) of life and property. 



The amount of deviation necessarily varies, because of the 

 unequal character of the disturbing force, not only in different 

 vessels, but in the same vessel in every change of magnetic dip, 

 and on every change of course. In very high magnetic lati- 

 tudes, the deviation may be such as to influence the compass 

 more than the directive action of the earth ; but in equatorial 

 regions it will be generally so inconsiderable as to be of little 

 importance in practical navigation. On two or more points of 

 the compass, the deviating force being coincident in direction 

 with the earth's magnetism, is not observable (these are deno- 

 minated " the points of change ") ; whilst its influence rises to a 

 maximum in the ratio of the sines of the course (nearly) on 

 both sides of the points of change. Most commonly the points 

 of change will be found to occur when the direction of the ship's 

 head is north or south, or nearly so ; and the maximum on or 

 near an east or west course. But to this rule there are many 

 exceptions. 



• From a " Lecture on the Deviation of the Compass," delivered at the 

 Royal Institution at Liverpool, by Captain (now the lleverend) William 

 Scoresby, F. R. S. ; January 23. 1822. 



