On the casting of Specula for Beflecting Telescopes, 1 8 



o»- ' " •-■ • "-'r • ~- -- --- - -- - - 



Ho Af»= — ~ — > ii^nittion io amiJ &ih m i»nlK/ afij snilon 



9W i.O>?,f. orb <l biiH ;9iiiJ Uv;£>wy., ..t'j»?& no i3i 



which is Mr. Milne's formula ; and similarly, it may be shown 

 that (6.) and (11.) coincide with the expressions given in 

 Milne. 

 Also if P be equivalent to an annuity certain of ^ years, so 



\ ^t \ 



that P= } and therefore p=i;'+', then (12.) becomes ' 



. a+b + c + «.v 



which is Mr. Milne*s expression. 



The formula (13.) is not in Milne, but is given by Professor 

 De Morgan in his Essay on Probabilities (Cab. Cyclop.), 

 Appendix the Second. It is very well adapted to computation, 

 though not I think quite so well as (12.), if Jones's tables pre- 

 viously alluded to be used. The difference between the two 

 formulas in point of practical application is however very 



III. On the supposed Inversion of Hydrostatical Principles 

 isohich takes place in the casting of Specula for Reflecting Te- 

 lescopes 071 a ^^ chilling" base formed of hoop-iron packed 

 edgewise. By Professor Potter, A.M.* 



THE method of forming the " chilling " base of the mould 

 for casting specula with hoop-iron packed edgewise, in- 

 stead of a simple disc of iron, is Lord Rosse's undisputed dis- 

 covery. In his paper in the Philosophical Transactions for 

 1840, entitled " An account of Experiments on the Reflecting 

 Telescope. By the Right Honourable Lord Oxmantown, 

 F.R.S," he has the following: — " At first a simple disc of iron 

 was tried, but although the castings were sound, there was 

 this defect; that bubbles of air were often entangled between 

 the iron and speculum metal, producing cavities which it was 

 tedious to grind out : the iron disc was therefore replaced by 

 one made of pieces of hoop-iron placed side by side, with 

 their edges up, tightly packed in an iron frame : the edges 

 were brought to a smooth surface of the proper curve either 

 by the file or lathe, whichever was the most convenient. A 

 metallic surface was thus constructed everywhere porous ; as 

 however close the hoop-iron had been packed, the interstices 

 suffered air to pass freely through. So successful was this 



♦ Communicated by the Author. 



