682 EAEL OF EOSSE ON THE CONSTETJCTION OF SPECULA OF 6-FEET APEETUEE, 
and the progress made is necessarily so very slow, that I think it would be inexpedient 
longer to keep back this paper in the distant hope of making it in some respects more 
complete. 
As to the instrument, a slight description of it has already been given in the ‘ Trans- 
actions’ for 1850, but without details, and in the ‘Transactions’ for 1840 the process 
employed in the construction of specula of 3-feet aperture was fully explained ; but in 
passing from specula of 3-feet aperture, and about twelve hundredweight, to specula of 
6-feet apertm’e and four tons, although the same principles were our guide, difficulties 
were encountered which called for new contrivances and additional precautions. It 
will, I think, be useful to give a short account of the process by which the 6-feet specula 
were made, and some details as to the mounting, supplying at the same time the best 
answers we can to the questions, so often put. What really are the optical powers of 
the instrument 1 What are the merits and demerits of its form of mounting, after an 
experience of more than ten years'? Would it be possible to construct a larger one, and, 
if so, would there be anything gained ? As there seems to be a desire to employ large 
instruments in different parts of the world, would it be possible to lay down instructions 
sufficiently precise to enable a mechanical engineer, without a previous apprenticeship, 
to undertake the construction of large instruments as a matter of business ? 
About one ton and a quarter of speculum-metal can be melted in one crucible ; and 
up to that weight there is no difficulty whatever in casting a speculum, and the instruc- 
tions in the ‘ Transactions’ for 1840 are amply sufficient to enable any engineer to do so. 
Each time, however, the crucible increases in circumference from the pressure of the 
metal, and after seven meltings we found the increase to amount to 4 inches. One ton 
and a quarter is therefore about the limit for a separate melting, and for larger specula 
we must employ several crucibles. The tin and copper must be previously combined in 
smaller crucibles, holding not more than three or four hundredweight, as the heat 
requhed is much greater than in the second melting. Three crucibles were employed 
in casting the 6-feet specula ; and we proceeded thus : — 
The crucibles (which had been cast by Messrs. Dewer of London, with the precau- 
tions detailed in my former paper) were placed in three separate air-furnaces upon cast- 
iron stands about 8 inches deep, and of somewhat larger diameter than the crucibles, to 
protect them from the immediate current of air passing through the fire-bars. A brick 
pillar from the bottom of the ash-pit relieved the furnace-bars from the weight they 
would otherwise have had to sustain. The furnaces were round, 4 feet diameter, and 
6 feet deep to fire-grate, — constructed as an ordinary air-furnace, with a door at the ash- 
pit to regulate the admission of air. The three furnaces were worked by one stack. 
In heating a crucible, it is necessary that the temperature should be raised gradually, 
beginning at the mouth ; otherwise it will be very liable to crack. To satisfy this con- 
dition, the crucibles (of course empty) having been placed on their supports, and the 
fui’naces filled with good peat, the fires were lighted at the top ; and in about ten hours 
the crucibles were of a proper temperature for the reception of the speculum-metal, 
