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speculum-metal, as it is, in tlie case of that which is 

 tough and mallealilc: only that, as the former, in cooling, 

 arrives sooner at its natural state of hardness and brit- 

 tleness, its external solid sliell will not bend, but break, 

 and fall into the void part vuidcr it; and thus form 

 cracks, or abrupt chasms, in the places, where tougher 

 metals would contract only regular depressions. And 

 also, when the body of the cast is small, or the mould is 

 so damp or cold, as to congeal, not only the surface, but 

 the substance, of the cast too soon, and thus prevent 

 a gradual influx of the fluid metal, to keep the central 

 part as distended, as the exterior shell was, when it be- 

 came fixed; the farther contraction of the interior parts 

 of this brittle, refractory metal, after it has become solid, 

 will be apt to form rents in it, because its substance will 

 not bear extension, without rupture. 



It woidd be an obvious remedy of the above incon- 

 venience, if there could be contrived a reservoir of fluid 

 metal, to descend into the intei'ior part of the cast, and 

 fill up the void made in it,>' las fast, and as long, as it is 

 forming by the contraction of the metal. Now, this is 

 effected, by having a jet or appendage to the cast, of 

 such a size, form, and position, as will be efl^ectual to re- 

 tain the metal, composing it, in a state of fluidity; and 

 also to suffer it to descend into the interior of the cast, 

 until all parts of the same become fixed, and dncapable of 

 receiving any farther influx of metal. For thus, all the im- 

 perfections, that would otherwise be in the cast itself, will 



now 



