of the Specula of the Reflecting Telescopes, 257 



To pass from these points, therefore, the object to which I 

 am desirous of especially calling the attention of those who 

 may be interested in this question, is the process of annealing 

 the metal after casting, since it is in this, as far as I have 

 found, that the chief disappointments take place ; and in con- 

 sequence of a species of mismanagement which attention will 

 be able to prevent, when once the principle has been explained. 

 Here it is at least that I did fail myself, until I contrived to 

 examine and discover the cause ; and here it is that Mr. 

 Ramage has found the same difficulties, and made the same 

 failures — not likely, I trust, to happen often again, though, in 

 a process so delicate, certainty must not always be expected. 



I need scarcely say, that if this alloy were suffered to cool 

 rapidly, it would break, or might break, and that it requires to 

 be cooled slowly in the mould, or annealed ; this being true, 

 particularly of the larger metals, as there is not so much 

 hazard with respect to small ones. The process itself is simple 

 and easy : it would scarcely, indeed, be necessary to describe 

 it were it not for its frequently injurious consequences; and it 

 consists commonly in covering the metal in the mould with 

 hot ashes or cinders, and thus retaining it, often for a con- 

 siderable time, and at a high temperature. 



To state now what those failures are, before enquiring into 

 the cause, — I must first remark, that the fracture of speculum 

 metal ought to be that which mineralogists term flat conchoidal, 

 or resembling that of gun-flint; and the surface of the frac- 

 ture ought to be lucid and smooth, or like that of glass, utterly 

 free of granulation or roughness of any kind, and as polished 

 in reality, as it can afterwards be rendered on the tool. With- 

 out this, it is in vain to expect the greatest light which a 

 speculum is capable of giving, nor the purest colour; and 

 without this also, it will be found that the surface will be apt 

 to fail after long use, or in consequence of frequent exposure 

 to the atmosphere and vacillations of temperature. A spe- 

 culum metal, therefore, which does not thus come out of the 

 annealing, ought to be rejected ; it is not worth the subsequent 

 abour. 



It is true, that, as far as four inches diameter, a very mode-* 

 rate degree of care will generally insure success ; the difficulty 



