MECHANICS AND USEFUL ARTS. 79 



facture is somewhat likely to induce brittleness. There can hardly 

 be any continuity of fibre in a plate forged from masses of scrap 

 iron, perhaps of different qualities, each of different heats ; the na- 

 ture of the weld and its form, and the repeated cooling and re-heating 

 of the plate, are also adverse to its possessing great toughness. The 

 rolled plates have been found more uniform in quality and of greater 

 toughness than the hammered: and though the difficulties in their 



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manufacture are grave, there is no departure from the ordinary prac- 

 tice followed in making large plates for other purposes. The difficul- 

 ties which do exist are chiefly due to the immense weight and size, 

 and the intolerable heat of the mass, which must be dealt with while 

 at a welding temperature. 



The general size of the armor-plates required for the plated frig-: 

 ates now building in England is from fifteen to eighteen feet long, 

 from two feet six inches to three feet ten inches wide, and four and 

 one-half inches thick. The weight therefore of the finished plate 

 ranges from sixty to one hundred and ten hundred-weight ; and in 

 the unfinished state it comes from the rolls at eighty to one hundred 

 and forty hundred-weight. From three to four inches is cut off the 

 sides, and ten or twelve from each end ; and in this item of waste 

 the hammering process has an advantage over the rolling. 



The mode of manufacture of a five-ton plate is as follows : Bars 

 of iron are rolled twelve inches broad by one inch thick, and are 

 sheared to thirty inches long. Five of these bars are piled and rolled 

 down to a rough slab. Five other bars are rolled down to another 

 ronjjh slab, and these two slabs are then welded and rolled down to a 



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plate of one and one-quarter inches thick, which is sheared to four 

 feet square. Four plates like this are then piled and rolled down to 

 one plate of eight feet by four feet and two and one-half inches thick ; 

 and, lastly, four of these are piled and rolled to form the final entire 

 plate. There are thus welded up together one hundred and sixty 

 thicknesses of plate, each of which was originally one inch thick, to 

 form when finished four and one-half inches, making a reduction of 

 thirty-five times in thickness ; and in this operation from thirty-five 

 hundred to forty hundred square feet of surface have to be perfectly 

 welded by the process of rolling. It is not surprising that even with ! 

 the greatest care blisters and imperfect welds should exist and render 

 the plate defective ; this is the chief difficulty to be overcome, and a 

 very serious one it is ; and as the magnitude and weight of the plate 

 increase, so does also the liability to failure. The final operation of 

 welding the four plates of eight feet by four feet by two and one- 

 half inches is a very critical matter. To bring a pile of four plates 

 of these dimensions up to a perfect welding heat all through the 

 mass, without burning the edges and ends of the plates most exposed 

 to the fire ; to drag this pile out of the furnace, convey it to the rolls, 

 and force it between them, in so short a time as to avoid its losing the 

 welding heat, is a matter of greater difficulty than those unacquainted 

 with the work would imagine. The intensity of the heat thrown off 

 is almost unendurable, and the loss of a few moments in the convey- 

 ance of the pile from the furnace to the rolls is fatal to the success 

 of the operation. 



A pile of four plates, heated in a special furnace, is drawn out by 



