MECHANICS AND USEFUL ARTS. 71 



or nine times their breadth of beam, and this for two reasons : first, to 

 obtain an increase of speed by giving fine sharp lines to the bow and 

 stern ; and, second, to secure an increase of capacity for the same 

 midship section, by which the carrying powers of the ship are greatly 

 augmented. Now there is no serious objection to this increase of 

 length, which may or may not have reached the maximum. But, un- 

 fortunately, it has hitherto been accomplished at a great sacrifice to 

 the strength of the ship. Vessels floating on water, and subjected to 

 the swell of a rolling sea, to say nothing of their being stranded or 

 beaten upon the rocks or sand banks of a lee shore, are governed 

 by the same laws of transverse strains as simple hollow beams, like 

 the tubes of the Conway and Britannia tubular bridges. Assuming 

 this to be true, and indeed it scarcely requires demonstration, it fol- 

 lows that we cannot lengthen a ship with impunity without adding to 

 her depth, or to the sectional area of the plates in the middle along 

 the line of the upper deck. 



If we take a vessel of the ordinary construction, or what some 

 years ago was considered the best, 300 feet long, 41 feet 6 inches 

 beam, and 26 feet 6 inches deep, we shall be able to show how 

 inadequately she is designed to resist the strains to which she would 

 be subjected. To arrive at this result we shall approximate nearly 

 to the truth by treating it as a simple beam ; and this is actually the 

 case, to some extent, when a vessel is supported at each end by two 

 waves, or when, rising on the crest of a wave, it is supported at the 

 centre with the stem and stern partially suspended. Now in these 

 positions the ship undergoes, alternately, a strain of compression and 

 of tension along the whole section of the deck, corresponding with 

 equal strains of tension and compression along the section of the keel, 

 the strains being reversed according as the vessel is supported at the 

 ends or the centre. These are, in fact, the alternate strains to which 

 every long vessel is exposed, particularly in seas where the distance 

 between the crests of the waves does not exceed the length of the 

 ship. 



It is true that arvessel may continue for a number of voyages to 

 resist the continuous strains to which she is subjected while resting on 

 water. But supposing, in stress of weather, or from some other 

 cause, she is driven on rocks, with her bow and stern suspended, the 

 probability is that she would break in two, separating from the insuf- 

 ficiency of the deck on the one hand, and the weakness of the hull 

 on the other. This is the great source of weakness in wrought-iron 

 vessels of this construction, as well as of wooden ones, when placed 

 in similar trying circumstances. 



Changes in Progress. Having directed attention to the strength 

 of ships, and the necessity for their improved construction, we may 

 now advert to the changes by which we are surrounded and to the 

 revolution now pending over the destinies of the navy, and the 

 deadly weapons now forging for its destruction. It is not for us alone, 

 but for all other maritime nations, that these Cyclopean monsters are 

 now issuing from the furnaces of Vulcan ; and it behooves all those 

 exposed to such merciless enemies to be upon their guard, and to 

 have their Warriors, Merrimacs, and Monitors, ever ready, clothed 



