MAGNETIC OBSERVATIONS. 



201 



As the After Azimuth Compass was a very poor instrument, the descrepancy 

 between theory and observation in the case of its deviations is not surprising. In 

 the case of all the other compasses, except perhaps the Forward and After Ritchie, 

 the agreement of the observed and computed values of the deviations is much more 

 satisfactory; and indeed the differences between them are so small as to be of very 

 little consequence for the ordinary purposes of navigation; still, viewed from a 

 purely scientific stand-point, they are larger than might have been expected. 



The hard and soft iron forces involved in the production of the semi-circular 

 deviation were next examined in order to ascertain whether or not their relations 

 to each other were such as to render it possible, in the case of ;i vessel swung for 

 the first time, to predict from the observed deviations of her standard compass what 

 the deviations would be at any other place. The coefficients of the semi-circular 

 deviation are 53 and (i, and the components of the hard iron force involved in their 



production arc — and \ • while the components of the soft iron force arc ° and • 



As these components act at right angles to each other, the total hard iron force 

 will be 



J 



a* ^ x 2 '■ 



and if we let a represent the direction in which it acts, measured from the ship's 

 head toward the right hand, we have 



tan a = 



P 



X 



In the same way the total soft iron force will be 



J 



?? "•" X 2 



and to determine its direction we have 



tan a ' 



By means of these formula; the following table was computed. It shows the 

 amount and direction of the hard and soft iron forces acting on each compass on 

 November 1, 1865, and June 23, 18G6. 





Hard Iron Force. 



Soft Ire 

















Compass. 



November I, iSfij. 



June 23,- 1S66. 









Amount. 



Direction. 



Amount. 



I inn Hon. 



Amount. 



Direction. 



Admiralty Standard 



O.460 



ooo°.8 



0.226 



34 8 .o 



0.024 



3S 6 °-i 



After Binnacle .... 



O.664 



000.2 



O.639 



35 3-o 



O.OIO 



240.4 



After Ritchie .... 



O.780 



349-° 



0.4M 



354-o 



0.018 



16.3 



After Azimuth .... 



0-375 



186.8 



O.449 



173-9 



O.007 



in. 2 



Forward Alidade .... 



0.107 



277.6 



O.178 



267.3 



0.016 



184.2 



Forward Binnacle 



0-159 



33 J -9 



0.254 



280.1 



0.048 



187. 1 



Forward Ritchie .... 



0.376 



347-2 



0-387 



289.1 



O.022 



219.9 



