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and oppose the pressure of the atmosphere. When it is 

 lUshioned, as here directed, and made of a bulk equal to 

 a third or fourth pai't of the mass of the mirror, or even 

 a fifth or sixth part, when the mirrors are of large size, 

 there Avill ever be found, in the top of the prism, after 

 the metal is cast, a deep pit or cavity, which contained 

 the metal, that had ran down into the mirror, after the 

 outer shell of the mirror, and sides of the prism, had be- 

 come solid and congealed; and the mirror itself will be 

 found perfect, without any sinking or cavity; which could 

 only be formed Joy an injudicious disposition of the jet or 

 appendage, 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 made 

 to each of them. 



In tliis manner I have (without a failure in any) cast many 

 min'ors 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 

 then- outer edges, suddenly forming a solid incompressible 

 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 bng thin ingots or bars; which, 



by 



