688 EAEL OE EOSSE ON THE CONSTEUCTION OF SPECULA OF 6-FEET APEETUEE, 
especially where the specula were of considerable thickness. To explain this, it is only 
necessary to remark that, when metal enters the damp-sand mould, heat will be imme- 
diately abstracted from it ; and as it rises in the mould by each successive addition of 
hot metal, it will somewhat dry the upper sand-surface before the metal reaches it. 
When, therefore, the mould is full, the lower' surface will be cooler than the upper; 
this does not happen when the mould is dry. In casting very thin specula, there is no 
time for successive actions ; the upper and lower surfaces solidify simultaneously, and 
there is a tendency to separation in an intermediate plane. In some cases the separa- 
tion was so complete, that a slight concussion actually divided the speculum into two 
discs. In discs of brass there is often, from the same cause, a very thin plane of porous 
metal running through the centre ; and where this occurs in the plate of an air-pump, 
a bouching is inserted to cut olf the communication between the external air and the 
central aperture. Keeping these facts in view, it would be naturally expected that by 
employing very open sand, as damp as possible, for the lower surface of the mould, and 
dry sand for the remainder, the best results would be obtained : and such was the case ; 
and where other means are not at hand, specula of 10 or 12 inches diameter can thus 
be easily obtained, provided they are of considerable thickness. This device, however, 
was not successful when we endeavoured to procure thin plates to face the compound 
speculum described in the ‘Transactions’ for 1840; the solidification of the upper and 
lower surface was too nearly simultaneous, and therefore there was irregular contraction : 
consequently a metallic surface was employed, from which the plate was removed the 
moment it was solid. 
There is yet another method of procuring excellent specula of moderate dimensions, 
which was dismissed, perhaps, in too summary a manner in the account of experiments 
in the ‘Transactions ’ for 1840. It has this to recommend it, that it can be carried out 
by persons who have had no experience in the management of melted metal ; and it is 
desirable to smooth the way, as far as possible, for beginners, who may, perhaps, by early 
success, be induced to proceed further. A cast-iron mould can easily be made at any 
foundry ; it must be at least two and a half times as deep as the required speculum ; it 
is to be placed in a temporary air-furnace, resting, like a muffle, upon two very strong 
deep bars, and is to be made perfectly level. The grate should be made of moveable 
bars, which can be withdrawn at the conclusion of the process, so that the fire may fall 
into the ash-pit. If charcoal is employed, the draught will be sufflcient to produce 
the necessary heat without a chimney. The proper quantity of speculum-metal, in 
pieces, is then introduced, and the cover put on. It is important that no pieces of char- 
coal should get in before the metal is melted, as they will often be found in the face 
of the speculum. As soon as the metal is melted, the cover is taken off, and the move- 
able bars are drawn out. The metal is then stirred with a broad flat tool, passed every- 
where over the surface of the mould to detach air-bubbles, and without loss of time a 
jet of water is thrown against the bottom of the mould, through a rose with exceedingly 
small holes, and distributed evenly. The action of the water must be suspended the 
