THE BRITANNIA AND CONWAY TUBULAR IRON BRIDGES. 
333 
Besides these, it was necessary to select stations, external to the Bridge but in the 
prolongation of its central line, at which normal observations should be taken, to which 
those taken within the tubes should be referred. Good stations were found about 700 
yards from the Carnarvon Abutment and 600 yards from the Anglesey Abutment. 
The circumstances of the Conway Bridge, consisting of only one tube 140 yards long 
for each line of rails, were much simpler. It was only necessary to make observations 
at the terminal towers, called the Chester Tower and the Holyhead Tower, and in 
the middle of the length, for each of the parallel tubes. An external station was found 
at 700 yards distance from the Chester Tower; but none could be found at a greater 
distance than 80 yards from the Holyhead Tower. I do not, however, imagine that 
the action of the iron bridge was in any degree sensible at this distance. 
The positions of the tubes of the two bridges with respect to the Magnetic Meridian, 
and the places of the observing-stations, will be understood from the plans in Plate 
XVIII. 
For support of the instruments in the observations, remarking that the tubes of the 
Britannia Bridge vary in depth, in their interior, from 22 to 26 feet, I constructed a large 
wooden step-ladder, 12 feet high/with a flat stage for instruments on the top, supported 
by two legs striding apart to the breadth of 8 feet (embracing the rails of the railway), 
which were duly connected with the ladder by cross bars. This structure was found to 
be very firm. The various parts were so united by screws that they could be easily 
separated ; and the men who attended on the observers were practised in the mounting 
and dismounting of the ladder-stage, till it was found that they could entirely remove 
the instruments and stage in one minute of time. This command of the apparatus 
enabled the observers to make their observations with little disturbance from passing 
trains. For use in the tubes of the Conway Bridge, which are not so deep as those of 
the Britannia Bridge, the step-ladder was lowered. It was necessary, in most cases, to 
use the light of a lamp. 
The magnetic observations to be made at all the stations were : — of the direction of 
total horizontal force, of the magnitude of total horizontal force, and of the magnitude 
of total vertical force. They were thus conducted : — 
The direction of horizontal force was found by observing with a prismatic compass 
the apparent direction of the tube. From the land stations the direction of the tube’s 
axis was observed without difficulty, and this was taken as the undisturbed or normal 
azimuth. In the tube stations the disturbed bearing of the end opening (when its 
centre could be well estimated), or that of a weighted rope hung from the top of the 
end frame, was observed. Six observations were made at each station. It is to be 
remarked that, with this instrument, apparent azimuths increase in the direction 
N.E.S.W., and therefore, in observing a fixed mark, an increase of reading implies that 
the needle has turned in the direction N.W.S.E. 
The magnitude of horizontal force was found by use of a vibrating needle, the same 
which I used in the fundamental observations in the ‘ Rainbow ’ (Phil. Trans. 1839), and 
mdccclxxih. 2 Y 
