354 
GENERAL SIR EDWARD SABINE ON TERRESTRIAL MAGNETISM. 
The values of the Declination and of the Inclination, recorded by the several observers, 
being in themselves absolute determinations, require for their correlation no other cor- 
rections than those for secular change. It is otherwise, however, in the case of the 
values representing the Magnetic Force, prior to the introduction of the practice now so 
generally adopted of determining and recording the values of the Magnetic Force, at 
the respective base stations, in absolute measure. Antecedently to the improvements 
in method and apparatus introduced by M. Gauss, it was the general practice of mag- 
neticians to express their results in reference to the force at a Base Station represented 
by an arbitrary value, which in London was usually taken 1*372, or, as written by 
M. Gauss, 1372. The base stations of British observers were commonly in the imme- 
diate vicinity of London, and as such may be fitly represented by the Kew Observatory , 
where the absolute magnetic force corresponding to a definite epoch, and its variation 
by reason of secular change, have been for some years past carefully determined. In 
the Philosophical Transactions for 1863, Art. XII. p. 302, and in the British Asso- 
ciation Reports for 1861, p. 273, will be found the premises from which the following 
Table has been obtained, showing the (at least approximate) value of the Magnetic Force 
at the Kew Observatory in absolute measure in each year between 1830 and I860*', 
which values have now been substituted at the corresponding dates for the arbitrary 
value L372, the force at the other stations of the respective surveys being expressed 
proportionally : — 
1830, 
July. 10-27 
1840, 
July. 10-28 
1850, 
July. 10-29 
1860, 
July. 10-30 
1831, 
3 3 
10-27 
1841, 
33 
10-28 
1851, 
33 
10-29 
1861, 
33 
10-30 
1832, 
33 
10-27 
1842, 
33 
10-28 
1852, 
33 
10-29 
1862, 
33 
10-30 
1833, 
33 
10-27 
1843, 
33 
10-28 
1853, 
33 
10-29 
1863, 
33 
10-31 
1834, 
33 
10-27 
1844, 
33 
10-28 
1854, 
33 
10-29 
1864, 
33 
10-31 
1835, 
33 
10-27 
1845, 
33 
10-28 
1855, 
33 
10-30 
1865, 
33 
10-31 
1836, 
33 
10-27 
1846, 
33 
10-28 
1856, 
33 
10-30 
1866, 
33 
10-31 
1837, 
33 
10-27 
1847, 
33 
10-29 
1857, 
33 
10-30 
1867, 
33 
10-31 
1838, 
33 
10-28 
1848, 
33 
10-29 
1858, 
33 
10-30 
1868, 
33 
10-31 
1839, 
33 
10-28 
1849, 
33 
10-29 
1859, 
33 
10-30 
1869, 
33 
10-31 
In the general Table in which 
the observations collected in this 
memoir are arranged 
the primary classification is in zones depending upon latitude as already stated, the 
position within the zone being determined by the longitude : the name of the observer 
and the date of the observation are given in all cases. So far the several entries are 
simply statements of facts. The additional columns, — viz. those in which the endeavour 
has been made to assign corrections for the effects of secular change, in the brief 
intervals between the dates of the observation and the mean epoch (1842 - 5), — have been 
supplied (with two notable exceptions) upon the best judgment which a careful com- 
parison of the facts so placed in juxtaposition has enabled me to make. Generally 
* Since continued to 1869. 
