200 
LINES OF MAGNETIC DECLINATION IN THE ATLANTIC. 
General Table of the Declination in the Atlantic.— \t may possibly be found con- 
venient for the purposes of navigation, that the books which contain a compendium 
of the tables requisite to be used at sea, should include a general table of the mag- 
netic Declination at a certain epoch for a convenient number of geographical posi- 
tions, with auxiliary tables furnishing the means of readily computing the Declina- 
tion for intermediate positions, and for other years. The subjoined Tables, Nos. IX. 
and X., have been formed for the purpose of supplying what has frequently appeared 
to me a desideratum in this respect. No. IX. is a general table of the Declination in 
the Atlantic for January 1840, at the intersection of every fifth degree of latitude 
and longitude between 60° north and 60° south latitude, taken from the maps which 
accompany this memoir. Should this table be adopted in future editions of any of 
the very useful compendiums referred to, auxiliary tables may be readily computed 
and added, containing the factors in longitude and latitude for facilitating the cal- 
culation of the Declination corresponding to intermediate geographical positions; 
whilst by means of Table X. the values of the Declinations in Table IX. may be 
adapted approximately to any other year for which the Declination may be required. 
The numbers which it contains are the values of the annual secular change of the 
Declination, which being multiplied by the interval of years from the date to which 
the table corresponds {i. e. January 1840), observing to prefix the sign + to the 
interval (in years) if the Declination is required for a subsequent year to 1840, or 
the sign — if required for an earlier year, will give the correction to be applied for 
the difference of epoch. 
The values of the secular change in Table X. are derived from the comparison of 
the maps which accompany this memoir with the map of the Declination in 1787? 
published originally (with the observations on which it was based) in Hansteen’s 
Magnetismus der Erde, and republished in this country by myself in the Report of 
the British Association for the Advancement of Science in 1835. The table conse- 
quently represents the average annual secular change which has taken place in the 
fifty-three years antecedent to 1840. But it will be remembered that the secular 
change in any particular locality is by no means a constant quantity ; and, although 
over a large proportion of the area of the Atlantic, there is reason to believe that the 
annual change is still continuing at a rate which does not materially differ from the 
average of the last fifty years, yet there are parts, (as for example, in the vicinity of the 
British Islands, and of the Cape of Good Hope,) where the secular change has latterly 
been obviously undergoing a considerable alteration. Tables formed as these have 
been, will therefore require to be reformed from time to time ; the general table by 
fresh observations, and the table of secular change by a comparison of the maps 
founded on those observations with those now given. The time that may be allowed 
to elapse before the present tables are thus reformed will probably depend less on the 
interests of navigation and science, than on the degree of attention with which these 
interests may be regarded by the authorities of the Admiralty in times to come. 
