MAJOR SABINE ON TERRESTRIAL MAGNETISM. 
139 
In comparing the line of no dip, deduced from the observations now collected, with 
the same line in the map corresponding to the year 1825 , taken principally from the 
observations of Captain Duperrey*, the differences which appear are everywhere 
conformable to what is known regarding the secular changes of that element, and are 
within the limits of the probable effects of those changes. On the American side, the 
line in 1837 is slightly to the southward of its position in 1825 . On the African side, 
the alteration is in the opposite direction, and is greater, corresponding to the rapidity 
with which the node, or point of intersection of the line of no dip and of the geogra- 
phical equator, is in progress of translation to the westward. We may infer, with 
some degree of approximation at least, the rate at which this transference is taking 
place, from the observations of Captain Allen in December 1833 at the Ilha das 
Rolhas, at the southern extremity of the island of St. Thomas, compared with those 
which I obtained in May 1822 at Fernandilla, and at the Ilha das Cabras, both near 
the northern extremity of the same island. Captain Allen’s station w'as distant from 
mine about twenty-six miles in a north and south direction, which in that part of the 
globe is equivalent to a difference of about twice as many minutes of dip. By my 
observations there was no dip at the northern extremity of St. Thomas in May 
1822 ; and by Captain Allen’s observations made at its southern extremity in 1833 , 
allowing for the twenty-five miles distance apart of the two places of observation, 
the south dip at the northern extremity in December 1833 appears to have been 
about 7° S., showing a secular change of 7° in 1 1 ^ years, or on the average 39 ' in each 
year. Taking into account the angle which the lines of dip in that quarter make with 
the meridian, the intersection of the line of no dip and the geographical equator 
should be transferred to the westward, by a secular change of that amount, at an 
annual rate of half a degree. It would also follow, that in 1833 the line of no dip 
should be found, in or near the meridian of St. Thomas, about 3^° to the north of 
the position it occupied in 1822 ; and that this was very nearly the fact we may infer 
from Captain Allen’s observations at Fernando Po in 3 ° 45 ' N. latitude, which gave 
in December 1833 a dip of 0° 48 ' N. 
Messrs. Dunlop and Sulivan’s intensity results in the Atlantic Ocean are shown 
in Plate V., where also the isodynamic lines are drawn with the best approximation 
afforded by the observations hitherto obtained, including the present. Mr. Dunlop’s 
results are distinguished by inclined figures placed on the left of the station marks, 
and Lieut. Sulivan’s sea results by inclined figures placed on the right of the station 
marks : his land results are shown by upright figures. 
In passing from the northern to the southern hemisphere, Lieut. Sulivan found 
the intensity continually diminish until about the parallel of — 23 °, in long. 317 °, from 
whence, in his further progress southward, it again increased. In returning from the 
* Philosophical Magazine, February 1839. Represented by a dotted line in the isoclinal map in this com- 
munication. 
