334 THE ASTRONOMER ROYAL’S MAGNETICAL OBSERVATIONS IN 
which was subsequently lent to Mr. Rundell and Staff-Captain Evans for observations 
in the I * * 4 Great Eastern.' In consequence of the amount of vertical disturbance (to be 
mentioned hereafter), the needle was sometimes inclined to the horizontal plane so far 
as to render it necessary to raise the upper part of its case, but not so far as sensibly to 
change the time of its vibration. Three sets of vibrations, each set consisting of 20 
vibrations, were usually observed. It is perhaps proper to remark that the actions of 
the magnetic bodies surrounding the needle were such that they did not produce a 
variation of force depending on the variable position of the needle in its arc of vibration, 
and thus the vibrations give a legitimate measure of the horizontal force. 
The magnitude of vertical force was measured by use of a portable dip-instrument, 
Barrow 24, for the loan of which I am indebted to the courtesy of the Kew Committee. 
The dip-instrument is furnished with four needles, but only one (marked A 1) was used 
in these observations, and always with the pole B charged with red magnetism (not 
always dipping, as will be seen). Usually eight observations were made at each station, 
four being made by reversing the frame and reversing the needle-pivots on the frame, 
and four more by repeating that order. The poles were not reversed during the expe- 
riments, but they were reversed in some observations of dip at Greenwich. At Green- 
wich, before and after the experiments, and also at Bangor and Conway, the needle A, 
was compared with A 3 . The results were not perfectly accordant, and did not enable 
me to judge with certainty whether the needle Aj was sensibly out of balance ; and I 
am inclined to believe that the dips at the experimental stations may be uncertain to 
the extent of 5 ' or more. The dip was always observed in the plane of the apparent or 
disturbed magnetic meridian. 
The first observation of the entire series was taken at Greenwich on 1872, July 31, 
and the last was taken at Greenwich on August 1G. 
I now proceed with tabular statements of the observations and their results. In 
explanation of the azimuths, it is to be remarked that from station 1 to 10 the observers 
advanced northward along the eastern tube of the Britannia Bridge, observing the appa- 
rent azimuth of the northern opening; and then from station 11 to 20 returned south- 
ward along the western tube, observing the apparent azimuth of the southern opening ; 
to the latter series 180° will be applied in the following calculations. Similarly in the 
Conway Bridge, from station 24 to 27 the observers advanced westward along the 
northern tube; and by stations 28, 21, 22, 23 eastward along the southern tube: 180 
will be applied to the latter. 
