506 
LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
fleeting surface, and as the best means of effecting that, to solder them upon an alloy 
of zinc and copper, (in fact, upon a species of brass,) which should expand and 
contract in the same proportion as speculum metal. 
The experiment was tried repeatedly upon a small scale, and the difficulties which 
presented themselves successively surmounted, till at length a speculum of three feet 
aperture was completed, which bears distinctly a high power. 
The evil most to be apprehended in this construction, was unequal contraction and 
expansion in the different changes of atmospheric temperature ; that, however, seems 
to have been sufficiently guarded against, as I have never succeeded in detecting the 
slightest change of figure. It is therefore highly probable, that specula very much 
larger may be constructed upon this plan, and that, in fact, by repeating a number 
of operations, each on a scale perfectly manageable, we may obtain a reflecting surface 
the most brilliant, and at the same time as large as can ever be usefully employed in 
astronomical researches. I will now proceed with the details of the construction of 
the three-feet speculum. I had previously ascertained that an alloy consisting of 
about 275 of copper to 1 of zinc, gave the same contractions and expansions as spe- 
culum metal ; it was necessary, however, to repeat the measurement with care, using 
the identical materials which were to be employed in the construction of the specu- 
lum. The following simple contrivance for obtaining the relative contractions and 
expansions of a bar of speculum metal, and different alloys of zinc and copper, had 
been employed in the previous experiments, and was again resorted to. 
A bar was cast of speculum metal fifteen inches long, and one inch and a quarter 
square; similar bars, but only three-fourths of an inch thick, were cast of the alloys 
to be tried, containing a little more or less zinc than the proportions I have given. 
A piece of brass, consisting of 2 ?5 of copper to 1 of zinc, was also cast and soldered 
to the bar of speculum metal, as represented in Plate XX. figure 1, where A B is spe- 
culum metal, C D brass, and E F the bar of alloy, with a small excavation in the lower 
end fitted by grinding, so as to rest steadily on the hemispherical disc G ; a thin slip of 
brass was also soldered at the upper end of the bar of speculum metal, and the two 
bars made to fit neatly there, so that when brought together, a very fine line could 
be drawn across them with scarcely any troublesome parallax at the joint ; the whole 
was then immersed almost to the top in a tin vessel of water of the temperature of 
the atmosphere, and that vessel placed in another much larger, also containing water. 
Pieces of ice were then dropped into the outer vessel, so that the temperature of the 
whole was evenly and gradually brought down nearly to 32°; and a straight line, as fine 
as possible, was then drawn across both bars, and examined with a microscope to as- 
certain that it was perfect. The temperature was then gradually raised by pouring 
hot water into the outer vessel until nearly 212° had been attained, and the line was 
again examined with a microscope, and where the alloy had been made by mixing 
2*74 of copper with 1 of zinc, and the loss in melting amounted to y^th of the whole, 
the continuity of the line was not broken in that range of temperature ; according, 
