SG REFLECTING TILESCOPE: 



A pit or cavity will be constantly and necessarily formed In tlit 

 face of the cast, in that part of it which was last congealed ; 

 which cavity will commonly be larger or smaller, in proportion 

 to the quantity of metal in the cast. 

 ^-particularly The event will, in this respect, be the same with speculum- 

 meur^^"*"' "^<2tal, as it is, in the case of that which is tough and malle- 

 able : only that, as the former, in cooling, arrives sooner at its 

 natural state of hardness and brittleness, its external solid shell 

 will not bend, but break, and fall into the void part under it; 

 and thus form cracks, or abrupt chasms, in the places, where 

 touglier 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 centra! part as distended^ 

 as the exterior shell was, when it became 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, be- 

 cause its substance will not bear extension, without rupture. 

 Remedy : by a It would be an obvious remedy of the above inconvenience, 

 supply of the Jf there could be contrived a reservoir of fluid metal, to des- 

 * cend into the interior part of <he cast, and fill up the void 

 made in it, as fast, and as long, as it is forming by^the con- 

 traction 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 effectual to retain the metal, composing it, in a 

 state of fluidity ; and also to suflfer it to descend into the inte- 

 rior of the cast, until all parts of the same become fixed, and 

 incapable of receiving any farther influx of metal. For thus, 

 all the imperfections, that would otherwise be in the cast 

 'itself, will now exist only in the appendage to it, which is 

 a supernumerary part, to be afterwards seperated from it. 

 This appendage ought to be of the form of a prism, and as 

 nearly that of a cube, as the operation of moulding it in the 

 sanli will permit ; for, in this gross shape, the metal in it will 

 •—in the form ^^ ^-'^^ longer cooling. It should be connected with that part 

 of a prismafic of the mirror, which is uppermost in the flask, and joined 

 appendage. .^^ .^^ ^^ ^ neck, equal in thickness to the edge of the mirror, 

 (but so posited, that the face of the mirror may project a 

 little above it), and, in breadth, about twice the thickness, 

 This neck ought to be as sKort as possible, i, e. just so as to 



