of a tubular bridge took possession of his mind. This last project, destined to prove so successful, has been well compared to a beam along which a man scrambles when escaping from a fire. Stephenson was bent upon crossing the Straits; but as he could not build an ordinary bridge, when under such extraordinary restrictions, he resolved to span the waters with a huge makeshift in the shape of a hollow beam of iron. Each tube of the Britannia Bridge is literally a beam, so constructed that it combines the maximum of strength with the minimum of weight; in other words, it is a beam from which every portion of metal that does not add to its strength has been carefully removed.
We will now endeavour to explain the simple principle upon which a beam, whether of wood or iron, is enabled to support the weight imposed upon it.
For want of a few moments' reflection most people, in looking up at a common ceiling-girder, consider that its upper and lower parts suffer equally in bearing the weight of the roof; but these upper and lower strata suffer from causes as diametrically opposite to each other as the climates of the pole and of the equator. The top of the beam throughout its whole length suffers from severe compression, the bottom from severe extension, and thus, while the particles of the one are violently jammed together, the particles of the other are on the point of separation; in short, the differ-
