_T_ is a
door in front of the box, which when raised allows the air of the room
to be heated and to circulate through the radiator. By adjusting the two
valves _D_ and _T_, air of any desired temperature can usually be
obtained. Figure 16 (after Billings) shows an English device intended
for the same purpose. The valve _D_ in this case operates to admit air,
either through the radiator or to the space between the radiator and the
wall, in order to vary the temperature of the entering air. The valve
_T_ may be open or closed, and its position, together with that of the
valve _F_, determines the proportion of the room air which is reheated.

[Illustration: FIG. 16.--Ventilating device.]

The writer remembers one schoolhouse where these methods were used
successfully, the radiators being placed directly in front of the window
and inclosed at the back, sides, and top, except for an opening to the
outer air through the wall, properly controlled by a damper. In the
writer's own office the radiators are by the side of the window and are
boxed in, the connection being made with the outside air through a
wooden box entering under the radiator. This is an admirable method,
provided the radiator has sufficient surface to warm the fresh air
admitted.

Another excellent arrangement is to provide a narrow screen similar to
that used for protection against flies, but with the screening material
of muslin cloth instead of wire cloth. This muslin will break up the
current of air so completely that no draft is felt by persons sitting
even close to the open window.

_Position of inlet._

The inlet for fresh air, if connecting directly with the outside air,
should not be at the top of the room, since then the inlet would not
serve to admit air, but rather to allow the warm air of the room to
escape, and a burning match would inevitably show a draft outward
instead of inward.

Neither is it desirable to have the fresh-air inlet near the floor of
the room unless the entering air is warm, because cold air admitted will
flow across the floor and remain there, not disturbing the warm upper
layers. The effect then is not to improve the ventilation, but only to
chill the feet of persons sitting in the room. The position of the
window lends itself, therefore, to admission of fresh air, since it is
neither at the top nor at the bottom of the room, but at the level most
suitable for such admission.

_Foul-air outlet._

Very few houses hav