320 
STAFF CAPTAIN F. J. EVANS ON THE 
Shetland Islands, and the N. and N.E. coasts of Scotland, the eastern coast of England 
from Holy Island to the North Foreland, and the N.W. and N.E. coasts of Ireland. 
The final results are generally the mean of two and sometimes three compasses, the 
individual observations having been made on several azimuths round the horizon, the 
true or astronomical bearings of the distant objects employed being obtained by an 
altitude azimuth instrument or theodolite of suitable size and telescopic power. 
The observations on the west coast of Scotland were made by Captain (now Admiral) 
Otter with an Adie’s Variation Instrument, in which three separate reversible needles 
were employed. The remaining declinations recorded, to which the observers names are 
appended, have received equal care in their determination with those above described. 
With the exception of a few stations, chiefly on the west coast of Scotland, for 
which the true or astronomical meridian had been furnished to the Admiralty Surveyors 
from the Ordnance Survey Office at Southampton, the astronomical meridian to which 
all the magnetic bearings were referred by the several observers was generally deter- 
mined in the following manner. 
With an azimuth and altitude instrument or the large class of theodolite employed 
in the Admiralty Coast Surveys (with azimuth circles of 5 to 6 inches diameter), — the 
zero of which was set to some well-defined object, — the sun’s exact altitude, together 
with the time of its centre passing the middle wire of the telescope, were noted, as also 
the reading of the azimuthal circle. With the exact latitude and longitude as obtained 
from data furnished by the Ordnance Survey Office, the sun’s astronomical bearing and 
also that of the zero-point of the instrument, together with the terrestrial object to which 
it was directed, were thus derived, by two separate methods, from well-known fonnulse. 
Azimuthal angles were then measured from the zero object to five or six well-defined 
landmarks, equally distributed, where possible, round the horizon; and these angles 
being referred to the astronomical bearing of the zero object, the astronomical bearings 
of the several landmarks from the instrument were thus known. 
The azimuth compass was now placed in the exact position of the azimuth and altitude 
instrument, and its sight-vanes directed successively to the several landmarks round the 
horizon, and their magnetic bearings observed, the mean value of the several differences 
between the magnetical and astronomical bearings being taken for the magnetic decli- 
nation [or variation of the compass] at the station. 
The observations have been finally reduced to the 1st January, 1872. For this pur- 
pose an arbitrary value (an assumed average free from diurnal change) has been assigned 
for the magnetic declination at Greenwich Observatory for that date, namely 19° 40' W. 
The differences between this assigned value and the recorded declination at Greenwich 
Observatory at the exact time * when the several observations were made on the coasts 
have been applied to the latter as corrections, and will be found detailed in the tabular 
* The Greenwich Magnetical Observations are published to 1868, and from this source the corrections to 
that date have been obtained. For subsequent comparative values I am indebted to the Astronomer Koyal and 
Mr. Glaisher, F.E.S., in charge of the Magnetic Department. 
