270 
GENERAL SIR EDWARD SABINE ON TERRESTRIAL MAGNETISM. 
applicable at Kew to the whole interval from 1834 to 1861. And from all these results 
we may infer the existence of a small hut tolerably well-assured progressive augmentation 
in the amount of the secular change of the Inclination, — 1°, with a diminishing latitude, 
and 2°, with an increasing westerly position. 
In the corrections to the mean epoch of 18 42' 5 I have employed in Scotland an 
annual decrease of 2'-0. In England, I have increased this rate with progressively 
diminishing latitude, on the eastern side to 2'*G5 in 51° 30', and on the western side to 
2'-85 in lat. 50° 30'. In Ireland I have taken 2'*3 in lat. 55°, increasing to 2'*8 in 
lat. 51°. 
C. Force . — In the determinations of the Intensity of the Magnetic Force, the Kew 
Observatory has been regarded as supplying the fundamental station of the British 
Survey. From the Philosophical Transactions, 1863, Art. XII. p. 302, we may assume 
the total force at Kew in absolute measure (British units), at the definite epoch of 
July 1, 1860, to have been 10*302 ; subject to an annual increase from secular change 
of -00125, as derived from the Kew Observations between April 1857 (when the regular 
series of absolute determinations at that observatory commenced) and March 1862. In 
accordance with these values the total force at Kew at the mean epoch (1842-5), for 
which the present maps are constructed, is taken as 10-280, and the corrections applied 
to the several determinations to reduce them to the mean epoch are proportional to an 
annual increase of -00125. 
The maps of the three magnetic elements which accompany this paper have been 
prepared at the Hydrographic Office, with the sanction of Admiral Richards, under the 
superintendence of Captain Frederick John Evans, R.N. 
