[ 331 ] 
VII. Magnetical Observations in the Britannia and Conway Tubular Iron Bridges. 
By Sir George Biddell Airy, K.C.B., P.B.S., Astronomer Royal. 
Received October 12, — Read December 19, 1872. 
It occurred to me some time since that it would be matter of interest to examine 
the character of the magnetic action of the iron in the Britannia and Conway Tubular 
Iron Bridges upon a magnetic needle within the tube. This was suggested to me by 
consideration, not so much of the mass and extent of the iron structure (although both, 
in the Britannia Bridge, are very great) as of the peculiar state of tremor to which the 
iron is continually subjected. After remarking, when within the tube of the Britannia 
Bridge, the disturbance of the iron produced by a passing train, my hand being firmly 
pressed against the iron wall, I described to the late Robert Stephenson my impression 
that the particles of the metal were in a state of “ molecular shiver,” and he replied 
that those words exactly represented his own idea on the agitation of the iron. 
All experiments appear to show that iron, in this state of tremor, is peculiarly subject 
to the inductive action of external magnetic force. When to this is added the consider- 
ation that the tubes have been unmoved in position, and that they have been subjected 
to this disturbance many times every day since their erection, it seems reasonable to 
conclude that they will exhibit the greatest amount of induced magnetism which it is 
possible for malleable iron to receive. I know not how far this susceptibility to mag- 
netic action may depend on the quality of the iron ; but I think it proper to state, on 
the authority of Mr. Edwin Clark, that the iron was made chiefly in Staffordshire and 
Coalbrookdale, a smaller portion in Derbyshire, and that it was the ordinary “ best-best” 
plates of the day, and intended to be scrap-iron throughout. 
My friend Mr. James Carpenter (then Assistant in the Royal Observatory) entered 
warmly into my views, and at my request undertook the conduct of the requisite 
observations; and I detached him for a few days (at my own pecuniary expense) 
from his duties at the Royal Observatory. Captain G. L. Tupman, R.M.A., who was 
at the time preparing himself in the use of instruments for observation of the 
Transit of Venus, gave his friendly assistance; and I am confident that the work 
undertaken by these gentlemen was executed with the greatest care and accuracy 
throughout. 
On my explaining my wishes to S. Reay, Esq., Secretary of the London and North- 
Western Railway, I was quickly informed that the Directors of the Railway had issued 
instructions that every assistance should be given by their officers ; and I was specially 
referred to J. O. Binger, Esq., District Superintendent at Chester, and Hedworth Lee, 
Esq., District Engineer at Holyhead. The observers, on their journey to Bangor, had 
interviews with these gentlemen, and with Mr. MacGuire and Mr. Macmillan, and 
