141 
in it. Both the prism and the mirror, therefore, ought to 
be filled by a lateral channel, opening (from the principal 
ingate) into the top of the prism ; which latter should be 
formed broad and flat, and not taper upward, like a pyra- 
mid, lest, by cooling where it grows narrow, it might form 
a solid arch and oppose the pressure of the atmosphere.. 
When it is fashioned as here directed, and made of a 
bulk equal to a third or fourth part of the mass of the mir- 
ror, or even a fifth or sixth part, when the mirrors are of 
large size there will ever be found in the top of the prism ? 
after the metal is cast, a deep pit or cavity which contain- 
ed the metal that had ran down into the mirror, after the 
outer shell of the mirror and sides of the prism had become 
solid and congealed ; and the mirror itself will be found 
ferfect, without any sinking or cavity ; which could only 
be formed by an injudicious disposition of the jet or ap- 
pendage, permitting the metal in it to freeze sooner than 
the whole mass in the mirror, and thus stopping its de- 
scent into it. If several mirrors be cast together, in the 
same flask, there must be such a separate appendage mad# 
to each of them. 
In this manner I have (without a failure in any) cast 
many mirrors of different sizes, and sometimes several of 
them together in one flask. But very small ones, such as 
the little mirrors for Gregorian telescopes, cannot be cast in 
this manner ; for their masses being but small, they cool 
too quickly to receive any additional infusion of metal ; 
and their outer edges, suddenly forming a solid incom- 
pressible arch, the central parts, in contracting towards it 
on every side, separate, and are rent asunder. And this 
has happened, even when I cast them in brass moulds 
made red hot : on which account I have been obliged to 
form them out of pieces of the metal, cast in long 
thin ingots or bass ; which, by nicking them across 
with a file, could be easily broken into square pieces, 
