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 tot he 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 consideration 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 magnetic 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.