294 POPULAR LECTURES AND ADDRESSES. 



it is certainly owing to the nearer agreement of the 

 former than of the latter with the period of the 

 ship's rolling, that in a heavy sea the Admiralty 

 compass is more disturbed than the ten-inch 

 compass in the merchant steamers. But to get 

 satisfactory steadiness a much longer period still 

 than the twenty-six seconds is necessary. Now, 

 for the same weight and dimensions of compass 

 card and needles, the smaller the magnetic moment 

 of the needles' magnetism the longer will be the 

 vibrational period. 



Hence, provided the bearing-point and cap be 

 fine enough and smooth enough to obviate serious 

 frictional error, greatness of magnetic moment 

 is a disadvantage in respect to steadiness of the 

 compass at sea. Smallness of magnetic moment 

 is important for another reason, which is, that 

 unless the magnetic moment be vastly smaller 

 than that of any of the compasses ordinarily in use 

 hitherto, the accuracy for all parts of the world, of 

 the correction of what is called the quadrantal 

 error in an iron ship, by the Astronomer-Royal's 

 method (to be explained below), is vitiated by the 



