TERRESTRIAL MAGNETISM. (APP. A.} 325 



those on which the forces were measured, amount- 

 ing to the same fraction of the radian (57*3) that 

 the difference of the measured forces is of their 

 sum. 



(4.) The difference of the sums of the directive 

 forces on opposite courses in two lines at right 

 angles to one another, divided by the sum of the 

 four forces, is equal to the proportion which the 

 quadrantal error, on the courses 45 from those on 

 which the observations were made, bears to 57'3* 



The deflector may be used either under way or 

 in swinging the ship at buoys. The whole process 

 of correcting the compass by it is performed 

 with the greatest ease and rapidity when under 

 way with sea room enough to steer steadily on 

 each course for a few minutes, and to turn rapidly 

 from one course to another. For each operation 

 the ship must be kept on one course for three or 

 four minutes, if under way, by steering by aid of 

 an auxiliary compass, otherwise by hawsers in the 

 usual manner of swinging at buoys, or by means 

 of steam-tugs. A variation of two or three 

 degrees in the course during the operation will 

 not make a third of a degree of error in the result 

 as regards the final correction of the compass. 

 The deflector reading is to be taken according to 

 the detailed directions in sections 14 and 15 of the 

 printed " Instructions." This reading may be 

 taken direct on the small straight scale in the 

 lower part of the instrument. The divided micro- 



