The boilers are formed of wrought iron. The material is previously rolled into plates of the requisite thickness, and then the first part of the process of forming these plates into a boiler is to cut them into proper forms. The monster that fulfills the function of shears for this purpose, bears a very slight resemblance to any ordinary cutting implement It resembles, on the other hand, as represented in the adjoining engraving, an enormous letter U, standing perpendicularly upon one of its edges. Through the centre of the upper branch of it there passes a shaft or axle, which is turned by the wheels and machinery behind it, and which itself works the cutter at the outer end of it by means of an eccentric wheel. This cutter may be seen just protruding from its place, upon the plate which the workmen are holding underneath. The iron plates thus presented are sometimes nearly half an inch thick, but the monstrous jaw of the engine, though it glides up and down when there is nothing beneath it in the most gentle and quiet manner possible, cuts them through, as if they were plates of wax, and apparently without feeling the obstruction. [Illustration: THE BENDING AND PUNCHING ENGINES.] The plates, when cut, are to be bent to the proper curvature. The machine by which this bending is effected is seen above, in the back-ground. It consists of three rollers, placed in such a position in relation to each other, that the plate, in being forced through between them, is bent to any required curvature. These rollers are made to revolve by great wheels at the sides, with handles at the circumference of them, which handles act as levers, and are worked by men, as seen in the engraving. The separate plates of which a boiler is composed are fastened together by means of massive rivets, and it is necessary, accordingly, to punch rows of holes along the edges of the plates for the insertion of the rivets. This process may be seen on the _left_ in the above engraving. Two men are holding the plate which is to be punched. The punch is driven through the plate by means of the great lever, which forms the upper part of the engine. The upright part in front is driven forward by means of the cam in the large wheel behind, a part of which only is seen in the engraving. This cam raises the long arm of the lever by means of the pulley in the end of it, and so drives the point of the punch through the plate. There is a support for the plate behind it, between the plate and the man, with a small opening in it, into which the punch enters, driving before it the round button of iron which it has cut from the plate. On the right, in the above engraving, is a punching engine worked by men, the other being driven by steam power. These machines are sufficient to make all the ordinary perforations required in boiler-plates. Larger holes, when required, have to be bored by a drill, as represented in the following engraving. [Illustration: THE BORING-ENGINE.]