Memoir to accompany a map of the magnetic variation for 1840 in the Atlantic Ocean between the parallels of 60° N. and 60° S. latitude, being contributions to terrestrial magnetism, No. 9

• Published in: Abstracts of the Papers Communicated to the Royal Society of London
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 1851
• Subjects: Antarctic; The Antarctic; Antarc*
• Online Access: http://dx.doi.org/10.1098/rspl.1843.0195; https://royalsocietypublishing.org/doi/pdf/10.1098/rspl.1843.0195

In this Number of the Magnetic Contributions the author gives maps of the Magnetic Declination in the Atlantic in January 1840, between the parallels of 60° N. and 60° S. lat., founded on 1480 determinations, by observers of different nations, all comprised between the years 1828 and 1848 each determination being in the majority of cases a mean of several distinct and independent observations, and all reduced to the epoch of 1840, by the rates of secular change de­rived from a comparison of Hansteen’s map of 1787 with the present state of the phenomena. A considerable portion of the determinations thus co-ordinated having been obtained on board ships in which measures were take'n to supply the means of correcting the errors occasioned by the in­fluence of the iron which the ships contained, the author in an ac­companying memoir has discussed at some length the variable part of the corrections required for that purpose, being that portion of the correction which varies as a ship changes her geographical po­sition. He infers from the observations generally, and especially from those of H.M.S. Erebus during the Antarctic Expedition in 1839-1843, that the disturbance occasioned by the iron is chiefly, if not entirely, due in wooden ships to the induced magnetism of the iron; but that its changes are not so rapid as those of the terrestrial mag­netic Inclination and Force during changes of geographical position, and that the magnetism of the ship is consequently at such times liable to become more or less in arrear if the expression may be permitted, of the change which the magnetic inclination and force have undergone; that in fact there are considerable portions of a ship’s iron which are not permanently magnetic on the one hand, nor perfectly soft so as to undergo instantaneous change with changes of inclination and force on the other hand; and which derive mag­netism by induction from the earth, but conform gradually rather than instantaneously to the changes of terrestrial magnetism corre­sponding to changes ...