ch (Fig. 60, B),
so that it will rest on the iron plate, and half fill the notch. The
aperture thus left forms the port of ingress for the hot metal (see
Fig. 61, M). A bit of sheet iron is attached to the upper surface of
the ring, and lies as a sort of flap, shaped like a deep shovel,
against the outside of the ring overhanging the port (Figs. 59 and 61
at F). This flap does not quite reach the iron plate, and its sides
are bent so as to be in contact with the ring. A portion of a smaller
ring is then applied in such a manner as to form a pouring lip or pool
on the outside of the main ring at E, and the metal can only get into
the main ring by passing under the edge of the flap and up through the
port. This forms an efficient skimming arrangement. The process of
casting is carried out by pouring steadily into the lip.

To avoid air bubbles it is convenient to cause the metal to spread
slowly over the chill, and Mr. Nasmyth's method of accomplishing this
is shown in the figure (61). The chill rests on three pins, A B C
(Figs. 59 and 61). Before pouring begins the chill is tilted up off
C by means of the counterpoise D, which is insufficient to tilt it
after the speculum is poured. It is important that the chill should
be horizontal at the close of the operation, in order that the
speculum may be of even thickness throughout. This is noted by means
of levels placed on the ring (at K for instance).

Fig. 61.

This apparatus may appear unnecessarily complex, but it is worth while
to set it up, for it makes the operation of casting a speculum fairly
certain. If the metal is at the right temperature it will form a
uniformly liquid disc inside the ring. The mass sets almost directly,
and as soon as this occurs it is pushed to the edge of the plate and
the metal in the lip broken off by a smart upward tap with a hammer.
The dovetailed bit of iron is knocked downwards and falls off, and the
ring may then be lifted clear of the casting. The object of the
dovetail will now be understood, for without it there is great risk of
breaking into the speculum in knocking the "tail" off.

A box of quite dry sawdust is prepared in readiness for the process of
annealing before the speculum is cast. The box must be a sound wooden
or metal box, and must be approximately air-tight. For a speculum a
foot in diameter the box must measure at least 3 feet both ways in
plan, and be 2 feet 6 inches deep. Half the sawdust is