PHILOSOPHICAL TRANSACTIONS. 
XII. Contributions to Terrestrial Magnetism. — No. IX. 
By Lieut. -Colonel Edward Sabine, R.A., For. Sec. R.S. 
Received May 24, — Read June 21, 1849. 
Containing a Map of the Magnetic Declination for 1 840 in the Atlantic Ocean between 
the parallels of 60° North and 60° South Latitude. 
In compliance with repeated representations from the Hydrographer of the Admi- 
ralty, that a correct map of the magnetic Declination over the Atlantic Ocean 
corresponding to the present epoch was most urgently required for the purposes of 
navigation, I have deemed it proper, — partly on account of the importance of the 
object itself, and partly in acknowledgement of the claim which the practieal wants 
of those who traverse the seas have on that physical science which they so much 
contribute to advance, — to suspend the progress of the publication of the observations 
made at the colonial magnetical and meteorological observatories, until in compli- 
ance with the wishes of the Admiralty a Declination map of the Atlantic has been 
completed. I have endeavoured at the same time so to conduct and arrange the 
preliminary investigation, that it may form a fitting part of the magnetic survey of 
the globe, which is designed to be comprehended in the series of Magnetic Contribu- 
tions, of which the first eight numbers have been honoured with a place in the Philo- 
sophical Transactions. 
The limits which have been taken for the map, in respect to latitude, are the parallels 
of 60° north and 60° south. The number of distinct determinations within those 
limits, either at sea or on adjacent coasts or islands, which have been reduced and 
coordinated, amounts to 1480. Each determination is, in the majority of cases, a 
mean result of several distinct and independent observations. They are all com- 
prised between the beginning of 1828 and the end of 1848; the commencement of 
1840 being taken as the epoch of the map; and each determination being reduced 
to that epoch by the rate of secular change derived by comparison with the map of 
MDCCCXLIX. 2 A 
