Page:Royal Naval Biography Marshall v4p1.djvu/32

 We mean good tough wrought iron; and had he not fallen into an error in principle in the construction, his invention, as applicable to naval purposes, would have almost defied the possibility of any further improvement.

“This gentleman was so confident of the correctness of his ideas respecting the superior strength, and consequent safety to be derived from iron chains in place of ropes, that he equipped a vessel of 400 tons, the Penelope, with iron rigging, stays, cables, &c. in which he proceeded on a voyage to Martinique and Guadaloupe, and in four months returned to London in perfect order, after experiencing every severity necessary to demonstrate the efficacy of iron in place of hemp. But the introduction of iron ground tackle, we consider as of much greater importance than any thing connected with the rigging.

“Since that time, iron cables have been introduced, not only into different ships in the royal navy, but in the merchant service, and with great success; for though some did give way in severe weather, especially of those first made, it is but justice to state, that even in these cases the hempen cables of the surrounding ships had all given way hours before, and that in most instances the ships so furnished have kept their anchors, when other ships parted and drove. This important fact has been so clearly established by reports from the different captains who have tried iron cables, that we confidently anticipate the day as not very distant, when hemp will be entirely discarded from the ground tackle of every British ship.

“We have said, that but for an error in his principle of construction, Captain Brown’s substitution of chain for hemp cables would have been perfect. This error arose from a prejudice natural to persons who are not thorough mechanicians, or who overlook those mathematical dicta which ought to guide every mechanical arrangement – an idea that a certain portion of elasticity should be given to the chain. To attain this ideal advantage, a certain degree of twist, equal to nearly one-fourth of a revolution, was given to each link; so that, when a strain comes upon the chain, it never finds any part of any portion of it in that situation which would present the greatest resistance to a change of figure. In such a chain, every strain makes an effort to bring every link into that form which it ought to have had in its first construction: and in proportion as the strain effects this, 80 far the links have been weakened, by having the particles of which they are respectively composed placed in a new order, at the expence of the corpuscular attraction exerted by these particles individually for those to which they are most contiguous.

“That this derangement of particles, and consequent diminution of strength, does take place in twisted links, is plain from what happens in proving the chains composed of such links: ‘a cable for a ship of 400 tons will stretch, during this operation, in a whole cable, nearly thirty feet!