This is but a small part of what promises more and more to become a great question—­that of navigation.  It is felt that, in these go-ahead days, we must be paying not less attention to our maritime than to our inland arm of commerce; and this has brought the question of wood versus iron ships again into prominent notice.  The advocates of iron shew that the dry-rot, so destructive to wood, cannot enter metal; that lightness and speed, those prime essentials, are insured by the use of iron; that iron ships are safer, more easily repaired, and cheaper than vessels built of wood; and that they are more lasting.  The chief objection hitherto has been the liability of iron to become foul in tropical climates; but this now appears to be in a measure overcome.  According to Mr Lindsay:  ’An admixture has been applied, termed “Anti-Sargassian Paint,” which has been found to answer the purpose better than any yet discovered.  From the experience of its properties, we cannot say that in itself it is yet sufficient; but it appears a fair substitute till some other preparation is discovered.  A gentleman at Glasgow,’ he adds, ’has already discovered a compound, which, being mixed in a fluid state with the iron, is expected to answer the desired purpose.  There is another disadvantage which will soon be overcome—­the greater liability to error in the compasses of iron ships; an error which, however, also occurs, though perhaps to a less extent, in every wooden ship.  By a most ingenious invention, which will shortly be made public, such errors in any ships, under any circumstances, can at all times be at once detected.’

An important patented process for producing tapered iron, has been explained before the Franklin Institute at Philadelphia—­one by which every variety of taper may be produced, or combinations of taper, with flat or other forms; and seeing how much tapered iron is used on railways, in many kinds of machinery, in ships and steamers, the subject may be considered worthy of more than a mere passing notice.  Tapered iron is a form to which machinery has been thought inapplicable, and only to be produced by hand-labour.  The new method, however, which has been successfully carried into practice at the Phoenixville Ironworks, is thus described:  ’The principle on which it acts is that of hydrostatic pressure, or, more properly, hydrostatic resistance.  A small chamber, similar to that of the common hydrostatic press, is set on the top of each housing; the closed end of the press being uppermost, and a plunger entering from below; but instead of water being forced into the press, the chamber is at first filled with water, and the pressure of the iron in passing between the rollers, tends to lift the top one, which is held down by the plunger.  An escape-pipe, provided with a valve, is inserted into the top of the chamber.  When any upward pressure acts on the top roller, it is communicated by the plunger to the water, which escapes through the valve, and the roller rises.