MAGNETIC OBSERVATIONS. 



201 



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

 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 llitchic, 

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

 satisfactory; and indeed the differences between them arc 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 a 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 33 and (2, and the components of the hard iron force involved in their 



production are . and --\ Avhile the components of the soft iron force are and f. 



A A A A 



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

 will be 



JaF+a? 1 



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

 liead toward the right hand, we have 



tan a. = 

 In the same way the total soft iron force will be 



C* ~ 



"ji 



A 



and to determine its direction we have 



. 



2 I 12 



tan a ' = 



c 



T 



By means of these formufa; 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, 1866. 



