80 REPORT — 1844. 



It appeared to me, therefore, to be an object so important to obtain a re- 

 fleeting surface which would reflect the greatest quantity of light, and retain 

 that property little diminished for a length of time, that numerous experi- 

 ments were undertaken and perseveriugly carried on. After a number of 

 failures, the difficulties appeared to be so great that I constructed three spe- 

 cula, where the basis of the mirror was an alloy of zinc and copper in the 

 proportion of 1 zinc to S-?^ copper, which expands with changes of tempera- 

 ture in the same proportion as speculum metal. This was subsequently plated 

 with speculum metal, in pieces of such size as we were enabled to cast sound. 

 These specula were very light and stiff', and their performance upon the 

 whole satisfactory ; but they were affected by diff'raction at the joinings of the 

 plates, and although very brilliant and durable, defining all objects well 

 under high powers, except very large stars, still as the effect of diflfraction 

 was then perceptible, they could not be considered as perfect instruments. 

 In the course of the experiments carried on while these three specula were in 

 progress, it was ascertained that the difficulty of casting large discs of brilliant 

 speculum metal arose from the unequal contraction of the material, which in 

 the first instance produced imperfections in the castings and often subsequently 

 their total destruction ; and it appeared evident that if the fluid mass could be 

 cooled throughout with perfect regularity, so that at every instant every 

 portion should be of the same temperature, there would be no unequal con- 

 traction in the progress towards solidification, nor subsequently in the trans- 

 ition from a red heat to the temperature of the atmosphere. Although it 

 was obvious that the process could not be managed so that the exact condi- 

 tion required should be fulfilled, still by abstracting heat uniformly from one 

 surface (the lower one), the temperature of the mass would be kept uniform in 

 one direction, that is, horizontally ; while in the vertical direction it would 

 vary in some degree as the distance from the cooling surface. These condi- 

 tions being satisfied, we should likewise have a mass which would be free 

 from flaws, and when cool would be free from sensible strain : nothing could be 

 easier than to accomplish this approximately in practice ; it would be only 

 necessary to make one surface of the mould (the lower one) of iron, of a good 

 conducting material, while the remainder was of dry sand. On trial this plan 

 was perfectly successful ; there was however a new, though not a very serious 

 defect, which was immediately apparent ; the speculum metal was cooled so 

 rapidly, that air-bubbles remained entangled between it and the iron surface, 

 but the remedy immediately suggested itself ; by making the iron surface 

 porous, so as to suffer the air to escape, in fact by forming it of plates of iron 

 placed vertically side by side, the defect was altogether removed. It only 

 then remained to secure the speculum from cooling unequally, and for that 

 purpose it was sufficient to place it in an oven raised to a very low red heat, 

 and there to leave it till cold, from one to three or four weeks, or perhaps 

 longer, according to its size. 



The alloy which I consider the best differs but little from that employed 

 by Mr. Edwards ; I omit the brass and arsenic, employing merely tin and 

 copper in the atomic proportions, namely, one atom of tin to four atoms of 

 copper, or by weight, 58*9 to 126*4. As it was obviously impossible to cast 

 large specula in earthen crucibles, the reverberatory furnace was tried, but the 

 tin oxidized so rapidly that the propoi'tions in the alloy were uncertain, and 

 after some abortive trials with cast iron crucibles, it was found that when the 

 crucible is cast with the mouth up, it is free from the minute pores through 

 which the speculum metal would otherwise exude ; and therefore such cru- 

 cibles fully answered the purpose. 



It was very obvious that the published processes for grinding and polishing 



