Cast iron was frequently used for the shell of boilers, with an internal fireplace and tubes of wrought iron, as shown in Fig. 8, and boilers of this construction are still to be found in use at some of the older works at the present day. As the outside shell and front plate are 1-1/2 inch thick and are not exposed to any wear at all, these boilers are sufficiently strong. A duplicate front plate with set of tubes attached is always kept on hand in case of need. Another form of cast-iron boiler is shown in Fig. 9., made in several parts put together with flange joints, with an internal fireplace and flue also made of cast iron. When cast iron was used for the parts exposed to the fire in boilers intended for high pressure, it was sometimes employed in the form of tubes of small diameter and proportionately thinner; as in Woolf's boiler, so much spoken of in the evidence before the parliamentary committee of 1817. This boiler, shown in Fig. 10., consisted of nine cast-iron pipes, about 1 foot diameter and 9 feet long, set in brickwork so that the flame played all round them. These small tubes were connected with another of larger size placed transversely above them, forming a steam receiver, and this again with a still larger one, which formed a steam chamber. No details of any explosions of the three last mentioned boilers have been obtained; but it is known that the cast iron was found a most treacherous material, especially when exposed to the action of the fire; and that the effect of explosion was very disastrous, because the boiler burst at once into many pieces, each of which was driven out with great velocity, and the danger was not mitigated by the circumstance of large masses holding together, as is found to be the case with wrought-iron boilers when exploded. * * * * * [Illustration: _Fig. 11._] [Illustration: _Fig. 12._]