THE ASTRONOMER ROYAL’S MAGNETICAL OBSERVATIONS IN 
explained fully the nature of the assistance which would he most useful to them. It 
was speedily arranged that the observers should be put in charge of Mr. Fletcher, 
foreman of the painters of the iron tube. It appears that it is necessary to give con- 
tinual attention to the painting of the iron ; anti the men employed for this purpose 
acquire a knowledge of the details of the structure and a facility of moving about all 
parts which are not possessed by any other persons on the railway. The selection of 
Mr. Fletcher to communicate immediately with the observers was therefore eminently 
judicious, and was attended with the best possible effect. Four of his men were in con- 
stant attendance on the observers, within the tube ; and to their faithful and zealous 
attention the success of the enterprise is mainly due. Policemen were stationed near 
the ends of the tube, and signalmen in proper positions, to give timely notice of the 
approach of trains ; and every person connected with the railway was evidently anxious 
to do his best to aid the party. I cannot too strongly express our obligations to the 
company and officers of the London and North-AVestern Railway for their cordial assist- 
ance in every part of the experiment. 
It was obviously unnecessary to examine the magnetic circumstances in proximity 
to the iron plat es (a condition which may be obtained anywhere), and I therefore 
determined on making observations solely in the very axis of each tube, as nearly as 
possible at the centre of its height and the centre of its breadth. The selection of 
points of observation, as regards the ordinate longitudinal to the tubes, was determined 
by their structure. The tubes and the land from which they start on both sides of the 
Menai Strait are more than 100 feet above the level of the sea, and from the brow on 
each side, from which the tubes start, there is a sloping bank to the water’s edge. At 
the foot of the bank on each side a tower is built ; there is also a central tower 
on the Britannia Rock, in the middle of the strait. Thus each tube consists of four 
parts having five supports, called respectively The Carnarvon Abutment, The Carnarvon 
Land Tower, The Britannia Tower, The Anglesey Land Tower, and The Anglesey 
Abutment. The lengths of the four parts are nearly 90, 170, 170, and 80 yards. The 
four portions of each tube were built and raised independently, the ends of each being 
strengthened with iron frames, for sustaining the strains which are incidental to the 
supporting ends of a tube ; but, after they were raised, the abutting ends were con- 
nected very strongly by riveted iron plates similar to those in other parts of the tube, 
and so arranged (by driving the rivets when the distant end of one of the tubes under 
the uniting process was somewhat raised) that there is a very strong tension in the upper 
part of the tube, at the place of junction, assisting to support the portions intermediate 
between two of the towers. The whole bridge consists, therefore, of two tubes, each entire 
from the Carnarvon Abutment to the Anglesey Abutment, but having greater quantities of 
iron at the ends and also on each of the three intermediate towers. It appeared to me, 
therefore, proper that observations should be taken at each of the five points of support 
(those at the two terminations being carefully taken in the transversal plane of the end 
of the iron work), and also in the middle of the length of each of the four partial 
tubes, making in all nine stations for each of the long tubes. 
