LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
507 
however, as the proportion of the zinc was more or less, the expansion of the brass 
bar was greater or less than that of speculum metal. 
As speculum metal and brass cannot be soldered together, except on a very small 
scale, with certainty by the ordinary methods practised in the arts, it may be useful 
to mention, that all that is necessary is, first, to fit the brass and speculum metal 
nicely, by filing or grinding, according to circumstances ; the brass is then to be 
tinned, and suffered to cool ; the surface of the speculum metal should be scraped 
lightly with a sharp chisel all over ; the two surfaces are to be placed in contact, and 
matters so managed, that a slight pressure may be applied after the fusion of the tin, 
and continued until it has again become solid ; the temperature should then be gra- 
dually raised till the tin melts ; and then, but not till then, resin applied in fusion, 
and also a little melted tin ; if resin or tin is applied in the solid state, owing to their 
rapid absorption of heat in becoming fluid, they will crack the speculum metal: the 
surfaces may be slightly separated, so as to ascertain that the speculum metal is 
tinned all over, which will be the case when the temperature reaches 400°: the whole 
must then be suffered to cool gradually. 
In casting the alloy of zinc and copper, some precautions are also necessary, not 
practised and not required in the arts. At the commencement of these experiments, a 
difficulty occurred which for some time I failed in overcoming ; in making the alloy of 
zinc and copper, and recasting it when made, much of the zinc was always volatilized, 
and the amount lost differed considerably each time, though the process was con- 
ducted as nearly as possible in a similar manner, so that it was impossible to allow 
for it, and the composition of the alloy was therefore uncertain ; a fatal objection to 
the whole process. The furnace was the ordinary brass-founder’s furnace, the air 
furnace of the chemist. I found, however, that the zinc was not volatilized in its 
metallic state ; its affinity for the copper was sufficient to prevent that, but it was first 
oxydized. To prevent the fresh air from beating down upon the crucible whenever 
the lid of the furnace was removed, the furnace was made much deeper ; this was of 
great use, but still the air which had passed through the fuel retained oxygen enough 
to act considerably on the zinc. Charcoal was next heaped over the crucible, still 
this was not effectual ; at length, charcoal in fine powder was tried in a layer two 
inches thick on the surface of the metal, occasionally renewed ; this was effectual. 
The crucible, therefore, must not be filled completely, and the charcoal dust can be 
conveniently thrown into it, whenever required, folded up in paper. If the process 
has been conducted with these precautions, the loss will be about y|-oth, and almost 
exactly the same each casting. 
The proportions of the zinc and copper having been determined, the brass work was 
first cast. By a reference to figures 2, 3, and 4, Plate XX., it will be at once seen in what 
way the materials were disposed of. Figure 2. represents one-eighth part of the whole 
seen in the reverse, a single casting ; fig. 3. the whole speculum, also seen in the 
reverse ; and fig. 4. the speculum seen on the opposite side previous to the soldering 
3 t 2 
