APP. N* IX.] MAGNETICAL OBSERVATIONS. 551 



8. A compass placed on either side of the ship's deck, 

 directly opposite to, or abreast of, the focus of attraction, 

 gives a correct indication on an east or west course, but is 

 subject to the greatest anomaly when the ship's head is north 

 or south ; and being here nearer the focus of attraction, the 

 anomaly is much greater than that observed on an east or 

 west course with the compass placed in the binnacle near the 

 ship's stern. 



a. This inference is founded on Observations No. 1, 2, 3, 

 8, 9, 13, 14, 15, 16. and 17. of the prefixed table. 

 The latter part of the inference, namely, that the 

 greatest anomaly occurs here when the ship's head is 

 north or south, is fully and uniformly established ; but 

 the former part rests only on the authority of Obser- 

 vations No. 8. and 9, though it derived additional sup- 

 port from several observations which I have exclu- 

 ded, because neither the sun, nor any other distant 

 object, calculated for proving the accuracy of the ob- 

 servations and determining the clear effect of the 

 " local attraction," was visible. 



9. A coinpass placed within six or eight feet of a cap- 

 stern spindle, or anchor, or other large mass of wrought iron, 

 foregoes, in a great measure, the influence of the focus of 

 attraction, and submits to that of the nearer body of iron. 



a. The effect of this is various, according to the relative 

 position of the compass and the iron. When the com- 

 pass is placed directly ahqft the body of ii'on, the in- 

 fluence is similar to, but greater than, that of the 

 focus of attraction on a compass placed near the stem, 

 as described in inference No. 6. (See Table of obser- 

 vations prefixed, No. 6, and 7.) When placed di- 

 rectly before it, the anomaly is similar in quantity, 

 but has its sign reversed ; and when placed on either 

 side of the mass of iron, the influence corresponds 

 more nearly with that of the focus of attraction on a 

 compass placed in the sides of a ship opposite to it, 

 as described in inference No. 8. A compass placed 



