MECHANICS AND USEFUL ARTS. 103 



to take a greater sweep of range than is possible where the parapet is of 

 masonry. In point of strength, an inch thickness of iron is equal to one 

 foot thickness of stonework, so that the power of resistance of the 

 shield in question is equivalent to that of a wall 15 feet thick. As a 

 matter of experiment it is to be put upon the parapet of one of the outer 

 ports at Cronstult, but should it be found to answer expectations, it will 

 itself take the place of the parapet, the whole metal platform being fas- 

 tened by cLimps and rivets into the granite rampart. 



The London Kn<jin,ecr thus describes the process of rolling the im- 

 mense bars of which this structure is composed. It says: As these bars 

 were an advance upon what has been hitherto done, the result was looked 

 forward to with some doubt, for each bar, when delivered, was to weigh 

 six tons, to be 15 inches square, to be tongued and grooved in the roll- 

 ing, and to be perfect in its soundness throughout. The furnaces were 

 opened at three o'clock, and the immense miss of metal was drawn forth 

 on to an iron truck, heated to a brilliancy that was almost blinding in its 

 intense whiteness, and instantly changing the temperature of the vast 

 factory to a scorching sulphurous heat that was insupportable. Directly 

 it was out, workmen, shielding their faces as they best could, swept the 

 impurities from its surface with long brooms soaked in water, but which 

 nevertheless lit like tow the instant they came in contact with the iron, 

 which was sparkling like a gigantic firework. It was then let down the 

 incline to where the rollers, turned by one of the largest fly-wheels in 

 the kingdom more than 10!J tons weight and nearly 40 feet in diam- 

 eter was waiting to crush the mass into its required form. This was 

 the critic d moment; for an instant or two the rollers failed to grip it, 

 but at last they caught it, and the whole machinery moved slower, as 

 amid loud cheers from the workmen they began to wind it in. As it was 

 slowly crushed through, the refuse melted iron was squirted out in all 

 directions, and as the mass emerged from the rollers on the other side, it 

 lit up everything with a bright lambent flame, said to be caused by the 

 pressure to which the bar was subjected. This was only the first roll, 

 but it had to be passed through three times to reduce it to the proper 

 thickness. It was not, however, as in the case of ordinary armor plates, 

 a mere question of reduction, as these bars have to be rolled, tongued, 

 and grooved to fit into each other. Thus in the rolling they have to 

 overcome all the peculiar difficulties of their construction almost in two 

 operations, which must be done while the metal is in a half melted state, 

 or the whole is spoilt. The bars, as we have said, are 15 inches square, 

 but each of these presents a most difficult section. In the first place, the 

 lower part of the bar has a projecting rib, and in the upper part is a 

 groove, corresponding in size with the rib on the lower half, so that the 

 projection of one bar may fit into the groove of the one beneath, thus 

 making a solid dovetailed wall of iron. Beyond these, also, is a rib at the 

 b.ick of the bar, formed to dovetail again into projecting masses of iron 

 in the rear supports of the fort, and in the process of rolling all these 

 departures from a plane and smooth surface have to be formed, and to be 

 formed with so much accuracy that each part fits into the other without 

 the necessity of any machine planing of surfaces. To give to the mass 

 of metal the required section, the rollers of the mill are grooved where 

 the raised surface is required, and sunk to produce the projecting ribs. 



