288 On the Anomaly in the Variation of the 



6. A compass placed near the stern, amidships of the quarter- 

 deck, is subject to the greatest anomaly or deflection from the 

 magnetical meridian, when the ship's course is about west or east; 

 because the focus of attraction then operates at right angles to 

 the position of the compass needle ; but the anomaly disappears 

 when the course is about north or south, because the focus of at- 

 traction is then in aline with, or parallel to^ the compass needle, 

 and conse(iuentlv has no power to deflect it from its direct po- 

 sition. [See Observations No. 4, 10, 1 1, and 12 of the prefixed 

 table.] 



This situation for the binnacle is deemed one of the best in the 

 ship, and is very properly preferred. Being abaft the focus of 

 attraction, the north point of the compas.s, in this magnetic he- 

 misphere, is always attracted forward, and the errors at equal 

 distances from the magnetic meridian, in the same dip, are alike 

 in quantity both on easterly and westerly courses, and always to- 

 wards the north ; the correction, when applied to the apparent 

 course, must therefore be towards the south, to give the true course 

 steered. Thus in high northern latitudes, where the anomaly is 

 great, (say 20°, or 10 degrees on each side of the magnetic me- 

 ridian,) a ship steering west bv the compass 100 leagues, and 

 then east 100 leagues, instead of coming to the place from whence 

 she started, will be 104 miles to the southward of it. 



7. The greatest anomaly with the compass in the position last 

 described, being ascertained by observation, the error on every 

 other point of the compass mav be easily calculated; the ano- 

 malies i)roduced by the attraction of the iron in the ship, being 

 found to be proportionate to the sines of the angles between the 

 ship's head and the magnetic meridian. 



Captain Flinders's rule is — As the sine of eight points (or ra- 

 dius) is to the sine of the angle between the ship's head and the 

 magnetic meridian (or sine of the course reckoned from south or 

 north), so is the anomaly found at cast or west bv observation, to 

 the anomaly on the course steered ; or, the anomaly on any other 

 course being found by observation, the error on that position of 

 the ship's head " would be to the error at east or west, at the 

 same dip, as the sine of the angle between the ship's head and 

 the magnetic me-idian, to the sine of eight points, or radius." 



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. 



This inference is founded on Observations No, 1, 2, 3, 8, 9, 



13, 



