TERRESTRIAL MAGNETISM, ETC. 295 



inductive influence of the compass upon the iron 

 correctors. Further, to allow the whole compass 

 error in an iron ship to be really well corrected, 

 without inconveniently or impracticably great 

 magnets and masses of iron fixed at inconveniently 

 great distances from the compass, the needles 

 ought to be not only of less magnetic moment, but 

 also much shorter than those in common use 

 hitherto. The double problem, then, of obtaining 

 a compass which shall be steadier at sea, and shall 

 also be better adapted for the perfect correction of 

 the error due to the iron of an iron ship, or of cargo 

 carried by the ship, requires 



1. For steadiness a very long vibrational period 

 with small frictional error. 



2. Short enough needles to allow the correction 

 to be accurate on all courses of the ship for the 

 place where the adjustment is made. 



3. Small enough magnetic moment of the 

 needles to allow the correction of the quadrantal 

 error to remain accurate to whatever part of the 

 world the ship may go. 



This problem forced itself on me when I tried to 



