Magnetic Needle as olserved on Ship-hoard. 289 



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 

 Observations No. 8 and 9, though it derived additional support 

 from several observations which I have excluded; because neither 

 the sun, nor any other distant object, calculated for proving the 

 accuracy of the observations and determining the clear effect of 

 the " local attraction," was visible. 



9. A compass placed within six or eight feet of a capstern 

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

 in a great measure, the influence of the focus of attraction, and 

 submits to that of the nearer body of iron. 



The effect of this is various, according to the relative position 

 of the compass and the iron. When the compass is placed di- 

 rectly abafi the body of iron, the influence is similar to, but 

 greater than, that of the focus of attraction on a compass placed 

 near the stern, as described in inference No. 6. [See Table of ob- 

 servation? prefixed, No. 6 and 7.] When placed directly 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 the ship opposite to it, as de- 

 scribed in inference No. S. A compass placed upon the drum 

 head of the capstern, any where out of the centre, will have its 

 Jiorth point so forcibly attracted by the upper end or south pole 

 of the spindle, that the' shi|)'s head may be made to appear to be 

 directed to any point whatever, at the pleasure of the experi- 

 menter. 1 have sometimes excited the astonishment of my offi- 

 cers by taking the binnacle compass and so placing it on the 

 capstern head, that the ship has appeared to be steering a course 

 directly contrary to that intended. 



10. ' When the iron in a ship is pretty equally distributed 

 throughout both sides, so that the focus of attraction occurs in 

 midships, a compass placed on the midship line of the deck (drawn 

 longitudinally) will be free from any anomaly from one end of the 

 ship to the other, when the course is north or south ; but on 

 every other course an anomaly will generally appear, increasing 

 as the angle between the ship's head and the magnetic meridian 

 increases, until the error is at a maximum, when the course is 

 east or west. 



The unequal distribution of iron in the ship, on board of whicli 

 I made all n)y cxiierimcnts, prevented the above effect from be- 

 ing realized. ' A blacksmith's shop was situated between decks, 

 on the larboard side of the fore hatchway. It was lined with 

 Vol. 54. No. 258. Oct. 1819. ' T sheet 



