144> Royal Astronomical Societr/. 



upper surface, which, receiving a stout iron hoop equal in hreadth 

 to the thickness of the speculum, formed an iron mould, and dispensed 

 altogether with the use of sand in the casting. The disc does not 

 require to be turned, but if cast from a well-made wooden pattern 

 will be sufficiently true ; neither do I think turning the hoop essential, 

 though it might be well to turn the inside surface and the edges, if 

 the means of doing so were at hand. 



"As it is necessary that the pouring should be pretty quick, in 

 order that there may not be time for the base to solidify any ])ortion 

 of the metal before it is completely covered, I inclined the base a 

 little, pouring on the lowest side, in order that the fluid might rise 

 in one compact wave ; and when the disc was nearly covered, it was 

 restored to a truly horizontal position, and the pouring continued, 

 until the mould was sufficiently filled, namely, to the depth of about 

 two inches and three quarters. The hoop was about three inches 

 broad, and having been turned parallel, the mould was in the first 

 instance placed horizontal, by a spirit-level being placed upon its 

 edge. The inclination was produced by the application of a lever, 

 which, when withdrawn, restored the base to its horizontal position, 

 and ensured the equable thickness of the speculum at every part of 

 its circumference." 



Mr. Lassell then describes the very ingenious method which he 

 adopted to procure the requisite quantity of metal in the proper state, 

 and his mode of ascertaining that the dose of tin was sufficient. The 

 final proportion which he used is 32 lbs. of copper to 15*09 lbs. of 

 grain tin, and 18 lbs. of white arsenic were stirred up with 438 lbs. 

 of the melted mixed metal. 



" The speculum was ground and polished on a machine almost 

 precisely the same as that described by Lord Rosso in his lordship's 

 very interesting paper, published in the second part of the Philoso- 

 phical Transactions for 1840. 



" I found, however, the grinding process much facilitated by in- 

 terposing a piece of sheet-lead, about a tenth or twelfth of an inch 

 thick, between the speculum and the iron grinding-tool. This saved 

 the rapid wearing down of the tool and also cut the metal much 

 faster, as the softness of the lead suffered the particles of emery to 

 imbed themselves into it, and thus to form a very keen grinding 

 surface. When the lead, fully charged with the emery, had become 

 smooth, it was exchanged for a fresh piece. When an entire sur- 

 face had been obtained upon the speculum, the smoothing and per- 

 fecting of the surface previous to polishing was produced by the iron 

 tool and the finest washed emery. 



" The speculum was polished many times on the same machine, 

 following as nearly as practicable the directions given by Lord Rosse ; 

 but, after several months' trial, I did not succeed in obtaining a 

 figure which satisfied me, the best I got being very inferior to the 

 surfaces I had obtained by hand on specula of various sizes, up to 

 nine inches diameter. In despair of success by this process, I 

 ultimately contrived a machine, in which I endeavoured to represent 

 as closely as possible the evolutions of the hand, by which I had 



