140 Specula . 
It would be an obvious remedy of the above incon- 
venience, If there could be contrived a reservoir of fluid 
metal, to descend into the interior part of the cast, and fill 
up the void made in it, as fast, and as long, as it is form- 
ing 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 effectual to retain 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 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 separated 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 sand will per- 
mit ; for, in this gross shape, the metal in it will be the 
longer cooling. It should be connected with that part of 
the mirror, which is uppermost in the flask, and joined to 
it by a 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 short as possible, i. e. just so as 
to permit it to be nicked round with the edge of a file, in 
order to break off the prism from the mirror when cast : 
for thus the heat of the large contiguous body of the prism 
will keep the neck from congealing ; which, if it happen^ 
ed, would stop the liquefied metal, in the prism, from run- 
ning down into the mirror. And, to prevent this, the 
prism ought not to form directly a part of the main jet or 
Ingate, by which the metal is poured into the flask ; for 
so the jet would cool sooner than the large mass of the 
mirror, and bear off the weight of the atmosphere, which 
ought to press on the fluid metal in the prism underneath, 
and force it down into the mirror, to fill up all vacuities 
