Magnetic declination

This is the angle between the magnetic meridian and the astronomical meridian. In Funchal the angle was 15° 30′ W. in 1768 (Dr Heberden); 21° 32′ W. in 1829 (Penny Cyclop.); 21° W. in 1835 (Wyld); 21° W. in 1843 (Vidal); 20° 57 1/2′ W. in 1851 (A. P. de Azevedo); 20° 56′ W. in 1861 (Friesach); 19° 55′ W. in 1885 (Admiralty chart); and 19° W. in 1892 (idem). According to the officers of the Challenger, the declination was 18° 25′ W. on 4 February 1872 and 20° 33′ W. on 17 July 1873. Colonel Francisco Afonso de Chaves, who also made magnetic observations in Funchal in 1903 (22 June) and in 1906 (4 June), found that in the first of these years the declination was 20° 31′ 30″ W. and in the second 20° 36′ 55″ W.

On 14 November 1842 the magnetic variation on Porto Santo was 24° 38′ 12″, according to the chart of that island by the late General António Pedro de Azevedo.

Bory, who visited Madeira in 1755 and published the account of his voyage in the Histoire et Mémoires de L’Académie Royale des Sciences pour l’an 1772, says that ‘the variation of the magnetic needle, which our position and the lack of a meridian line prevented us from determining on land, was often observed aboard the Cometa; it ranged between 10° 35′ W. and 11° 55″, which gives a mean result of 11° 3/4.’