LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
115* 
such an event is now taking place at the Cape of Good Hope. If we examine 
Erman’s map of the Declination in 1827-1830, published in the magnetic instruc- 
tions of the Royal Society, we find one of the separating lines in the neigh- 
bourhood of the Cape of Good Hope, and if we compare this map with those of 
earlier epochs, we find the position of that line progressively more and more to the 
east as we ascend in the order of time. Hence we should be led to expect that 
about this period it might be found to pass over the meridian of the Cape. The ob- 
servations which have been made daily at the magnetic observatory at the Cape, 
since its establishment in 1841, give reason to believe that the westerly declination 
which had been increasing for above two centuries, attained its maximum in the year 
1842 or 1843. In April 1841 the declination was 29° 05' west, in and April 1844 29° 06 , 
west*. The earliest observations at the Cape with which I am acquainted, are those 
of Davis in 1605, and Keeling in 1609. (Purchas, Book iv. ch. 6. § 1. and Book iii. 
ch. 6. § 4.) According to these observations the declination in 1605 was 0° 30' east, 
and in 1609 0° 12' west-j~. The line of no declination probably therefore passed over 
the Cape about the year 1607, and in 235 years the westerly declination has increased 
from 0° to 29°, (omitting the odd minutes,) or at an annual average rate of 7 , '4. Ob- 
servations at several intermediate epochs show that the progression of this change 
was at least not very far from being an uniform one. If we divide the whole period 
into four equal parts, we should have 
In the year 1607 
In the year 1666 
In the year 1725 
In the year 1784 
In the year 1843 
O I 
0 0 
7 15 W. 
14 30 W. 
21 45 W. 
29 0 W. 
In the appendix of Hansteen’s Magnetismus der Erde, p. 24, we have the record 
of actual observations as follows : — 
In the year 1667 
In the year 1 724 
In the year 1780 
7 15 W. 
(4 6 27 W. 
' ll6 18 W. 
. 22 16 W. 
We may therefore conclude that the westerly declination at the Cape, which for 
above 200 years had increased at an average rate of about 7^4 a year, or a degree in 
about eight years, has been for the last three years nearly stationary, having arrived 
at a maximum of 29° and a few minutes about the year 1843 ; and that a decreasing 
progression may now be expected Ships passing the Cape, on a voyage to the 
* The observations at the magnetic observatory at the Cape of Good Hope, preparing for the press, will 
show the mean declination in each month of the years referred to. 
f See also, for the latter observation, Hansteen’s Magnet, der Erde. Anhang. S. 146. 
t Captain FitzRoy observed 28° 30' in 1836 ; at that epoch, consequently, the maximum had not been 
reached. Sir Edward Belcher, in 1842, observed 29° 13'. 
MDCCCXLIV. 
R 
