NAVIGATION. 109 



careful steering by compass, we cannot trust to 

 making a course which will be certainly within 

 a quarter of a point of that desired. Now you 

 know an error of a quarter of . a point in your 

 course, would put you wrong by one mile to 

 right or left of your desired course for every 

 twenty miles of distance run. Thus in the most 

 favourable circumstances you are liable, through 

 mere error of steerage by compass, to be ten 

 miles out of your course in a run of two hundred. 

 In an iron ship, if the ordinary compass has been 

 thoroughly well attended to as long as the weather 

 permitted sights of sun or stars, a very careful 

 navigator may be sure of his course by it, within a 

 quarter of a point, when cloudy weather comes 

 on ; but by the time he has run three or four 

 hundred miles he can no longer reckon on the 

 same degree of accuracy in his interpretation of its 

 indications, and may be uncertain as to his course 

 to an extent of half a point or more until he 

 again gets an azimuth of sun or star. No doubt 

 an exceedingly skilful navigator may entirely, or 

 almost entirely, overcome this last source of 



