

ADDRESS, hi 
observations by three systems of Lines, usually called isogonic, isoclinal, 
and isodynamic lines. [Lines of equal horizontal direction, of equal 
inclination, and of equal force.] | In the maps of these lines existing 
in 1838, large spaces of the earth’s surface were either blank, or the lines 
passing across them were very imperfectly supported by observations. In 
the more frequented parts, where observations were more numerous, the dis- 
crepancies of their dates impaired their suitability for combination; for the 
position and configuration of the magnetic lines has been found to undergo 
a continual process of systematic change, with the causes of which we are as 
yet wholly unacquainted, but which has obtained the name of secular change 
to distinguish it from periodical variations of known and limited duration. 
Amongst the most marked deficiencies in these maps, were the greater part 
of the extra-tropical portion of the southern hemisphere,—the British pos- 
sessions in North America, and British India;—magnetic surveys of these 
were expressly recommended, and the practicability and advantage of 
making the observations on board-ship, and of thus extending them over the 
surface of the ocean, were pointed out. It is most pleasing to recall to recol- 
lection, and gratifying to acknowledge from this chair, the favourable manner 
in which the recommendations of the British Association were received by 
Her Majesty’s Government and by the East India Company, and how 
promptly and effectually they have been carried out. The blanks in the 
southern hemisphere have been filled up by maritime expeditions appointed 
expressly for the purpose. Magnetic surveys have been completed of 
British North America at the expense of our own Government, and of the 
Indian Archipelago at that of the East India Company, and India ‘itself is 
now in progress; whilst from the zeal of our naval officers contributions 
have flowed in from almost every accessible part of the ocean. The coordi- 
nation and mutual connection of so large a mass of materials is necessarily a 
work of time, but is progressing steadily towards completion, and when pre- 
sented in one connected view, will form the groundwork on which will securely 
rest a general theory of terrestrial magnetism corresponding to the present 
epoch. Until these combinations and calculations are performed, it would be 
obviously premature to speak of numerical values by which the magnetic 
forces at’ one part of the globe may be compared with those of another, or 
with forces of other descriptions; and for the same reason it is desirable 
to abstain for the present from notices of the geographical positions which 
particular lines, or as some may deem them, critical points in the magnetic 
resultants may occupy on the earth’s surface at the present epoch. Such 
notices could only be as yet provisional and liable to the amendments which 
more exact and extended calculation must be expected to produce. But 
thus much may be safely stated in reference to the general character of the 
three systems of lines which have been spoken of, that when derived afresh 
and exclusively from the observations of the last few years, they do most 
fully confirm the general conclusions derived from the observations of earlier 
