To construct an altar such that, when a fire is raised on it, figures at the side shall offer libations. Let there be a pedestal, A B C D, (fig. 11) on which the figures stand, and also an altar, E F G, perfectly air-tight. The pedestal must also be air-tight, and communicate with the altar at G. Through the pedestal insert the tube
H K L, reaching nearly to the bottom at L, and communicating at H
with a bowl held by one of the figures. Pour liquid into the pedestal
through a hole, M, which must afterwards be closed. Now if a fire be
lighted on the altar E F G, the air within it, being rare-fled, will
descend into the pedestal, and exert pressure on the liquid it
contains, which, having no other way of retreat, will pass through the
tube H K L into the bowl. Thus the figures will pour a libation, and
will not cease so long as the fire remains on the altar. When the fire
is extinguished, the libation ceases; and as often as the fire is
kindled the same will be repeated. The pipe through which the heat is
to pass should be broader towards the middle, for it is requisite that
the heat, or rather the vapour from it, passing into a broader space,
should expand and act with greater force.