bath resolves itself, primarily, into the fact that we have to
continually remove _the bottom layer of air_. The provision of the
foul-air conduits below the floor level is equivalent to providing a
suspended floor with a hollow space under. This is just the reverse of
the principle of ventilating rooms of ordinary temperature, where we
require to constantly remove the top layer, and often actually do so
when we provide false ceilings to passages, &c.

The ventilators placed at the floor level of the hot rooms should be
actually so, and not 3 in. or 6 in. above. Long, wide gratings 6 in.
deep are preferable to those of deeper and narrower design. In theory,
indeed, the whole circumference of the hot rooms should be lined round
with gratings, thus making the sudatorium like a lidless box inverted,
into which hot air is thrown and escapes all round the bottom edges.

There is one point about the circulation of air in a set of hot rooms
that requires considerable attention, and that is the _back-flow_ along
the floor. In any bath where hot air is supplied, if the bather will
hold his linen "check" across the top of the doorway between the rooms
he will find that the air is flowing from the laconicum to the
shampooing room. If, however, the sheet be held across the lower
portion of the doorway, he will find that there is a current of air
setting in an opposite direction--from the shampooing room to the
laconicum. This is shown at Fig. 11.

[Illustration: FIG. 11.

Longitudinal Section of Sudatory Chambers.]

It will be seen from the diagram that the bather is really in this
back-flow when he is standing between and in a line with the doors of
the hot rooms. All the air appears to be travelling along the top of the
bath, and the bather reclining on the marble-topped benches would seem
to be bathed in air that has passed along the top of the bath, round the
shampooing rooms, and back along the floor. In reality, however, it is
only from door to door that the currents exist exactly as shown at the
diagram, Fig. 11, there being a secondary circulating process in each
room.

This circulation of air will exist in any bath heated on the modern
system--that is to say, where freshly-heated air is passed in in
sufficient quantity. It is a natural result, and tends to distribute the
heat more equally. The back-flow is only objectionable when a door is
opened direct from the heated shampooing rooms to a cooler apartment, as
the