The making and holding of the core that forms the .steam jacket of a cylinder is an operation requiring great care. It can be made of loam or dry sand. It must be made about a cage, formed of rods and pipes. The jacket is usually cast with one end closed and four vent holes at the other. The diameter of the vent holes is equal to the thickness of the core. They are spaced at equal distances about the circle. The cage may be made by casting the rods and pipes inside of rings. The easiest method of casting the rings is to set the pipes and rods in an open sand mould and pour the metal around them as in Fig. 00. Reverse for the other end. The cage as finally made in shown in Fig. 01. The pipes are to be perforated for the escape of the gases. The dry sand is built about this cage covering rings, pipes and rods with the exception of one end of the pipes. These project from the core. The core, as thus made, is a hollow cylinder whose diameter and length corresponds to that of the jacket. This core is held in position by studs and chaplets. Fig. 62 shows the method of setting the core which is shown at A. It is supported by the chaplets a a, which are spaced at an angle of 120º around the circumference. Therefore those supporting the core do not appear in the section. The rear end of the core is steadied by the studs b b b, which pan through the dry sand of the mould and are steadied by the wedges. Tle Upper and lower wedges are reached by pockets left in the mould. The center one can be reached at the parting. At the other end the pipes are protected by the cores d d d. These cores also make the holes in the jacket end through which it is cleaned. The end face of the jacket mould is formed by the core e. This cure is made in sections so that it can be placed about the rods when the core is in position and held by the studs. The method of cutting these sections is shown in Fig. 63. The mould is now ready for pouring. When the metal is cooled the cage of the jacket core will remain inside the casting. It is removed by inserting a cold chisel through the vent holes, breaking the rings into small pieces and withdrawing them through the holes. The rods are buried in the sand of the core which must, of course, be removed before any of the iron work can be taken out.