I24 FOUNDRY WORK I; ingate and runner being one. Deep moulds are treated differently. The metal is either led in somewhere down the side, to lessen the height of its fall, or at the bottom, to rise quietly without any cutting action or splash. The pouring basins for small moulds are of simple cup-shapes, moulded in iron rings set on the top box, through which the metal, skimmed, passes directly into the mould. But when large moulds are poured from a ladle slung in the crane, some little time is spent in tipping and adjusting the spout of the ladle. The first driblets, therefore, are not permitted to fall directly into the mould, but into a deeper depression of the basin to one side of the runner. The metal then being poured into this in a full volume overflows into the runner, slag being kept back by the skimmer. When a mould is filled through several adjacent ingates, they are supplied from a common basin. Long moulds are poured from opposite ends, to avoid the chilling effects of a too- prolonged contact with the cold sand. No rule can be stated for the cross- sectional areas of runners and ingates. These have to vary with the degree of fluidity of metals and alloys. The only rule is to have them large enough to fill the mould before any chilling effect can occur. Also, the thinner the metal the more numerous must the ingates be made, until for the thinnest a spray of runners is used, fed from a common ingate. In castings of medium thickness, a runner of oblong section, much longer than its thick- ness, is used. Oblong runners are also better than large round ones for the fettlers, because they are more easily severed, and are less likely to cause a depression in the casting if broken off. Skimming chambers are provided when metal has to be scrupulously clean. Ordinarily metal is cleansed by " dead-melting ", and by baying back the scoriae in the ladle with the skimmer. When small articles have to be machined all over, centrifugal action is enlisted to send the heavier metal to the circumference of the chamber (fig. 17), whence it is directed into the mould, leaving the lighter impurities about the centre, to remain there or to float up into a riser. Risers and flow-off gates resemble the plain cup-shaped pouring basins, and their functions are (a) the relief of excess of pressure and strain on the top part of the mould, and (b) the discharging of an excess volume of metal with which dirt and air bubbles might have become entangled. Pouring is therefore continued for a short period after the mould has been filled. The relief of strain is important in the case of moulds having large areas. Fig. 17.—Skimming Chamber A, Chamber. B, Ingate. c, Riser.