MAGNETIC OBSERVATIONS. 



131 



On a piece of cardboard of suitable size a horizontal line a b / 5| inches long, 

 was drawn, and divided into eighths of an inch ; each half inch representing one 

 degree, and the whole line representing 11 15', or one point of the compass. 

 Touching the extremities of the line a b, and at right angles to it, were drawn the 

 line cd and ef; and each of them was divided, upward and downward from the 

 line a b. into points and eights of points; 1 each point occupying the space of 2{| 

 of an inch. Finally, a straight slip of drawing paper was divided on its edge into 

 degrees and sixths of a degree, each degree occupying a space of one-quarter of an 

 inch ; and the graduation was numbered from the middle towards each extremity. 



Then, to compute the values of <5 for any compass at any place, the paper scale 

 was laid down parallel to, and to the right of, c d, and at a distance from it (measured 

 on the line a 6) equal to TO; next, without moving the paper scale at all in the 

 direction a b, it was slipped up or down, as might be necessary, in the direction 

 parallel to c d, till the line a b cut the division on it which was equal to (m-\- L); 

 the zero of the scale being above the line a b if (m -|- L) was negative, below it if 



1 For computing the deviations of the Admiralty Standard and After Azimuth Compasses the 

 lines cd and ef were divided into degrees and sixths of a degree, each degree occupying the space 

 of one-quarter of an inch. 



