lf of the circumference, and afterwards expanding gradually
for discharging into the base of the chimney, the air from the tunnel
being drawn in at the center of the fan at each side, and discharged
from the circumference of the fan by the revolution of the vanes.
The engine driving the fan is started by telegraph signal at each
departure of a train from the terminal station, and the fan is kept
running until the discharge from it becomes quite clear, showing that
no steam or smoke remains in the tunnel; this is usually the case in
about eight minutes after the time of the train entering the lower end
of the tunnel, the passage of the train through the tunnel occupying
about three minutes. The fan draws air in at both ends of the tunnel
simultaneously, and begins to clear the lower end immediately upon the
train entering; the clearing of the upper end commences as soon as the
train has passed out of the tunnel, and as the fan is situated nearer
the upper end of the tunnel than the lower, the clearing of both
lengths is completed almost simultaneously. The fan is so constructed
as to allow an uninterrupted passage through it, for the air, whilst
the fan is standing still; and the natural ventilation thus obtained
by means of the large chimney is found sufficient for clearing the
tunnel during the night and some portion of the day, without the fan
being worked at those times. This natural ventilation is aided by the
engine exhaust and the boiler discharging into the chimney. The fan
has now been in regular operation for three-quarters of a year, and
has been found completely successful.

* * * * *

IMPREGNATING WOOD WITH TAR OR OTHER PRESERVING MATERIAL.

The preservation of wood is a problem which is attracting increased
attention, as year by year diminishes the material supply of timber,
and consequently gradually increases its price. Among other methods
employed, the impregnation of wood by the vapors of tar, creosote,
petroleum, etc., has been tried, and one of the practical difficulties
met with has been the obtaining of suitable apparatus for the purpose.

[Illustration]

The engraving annexed is an invention intended to supply this want.
The wood is inclosed, in a tank kept hot by a steam jacket which
surrounds it, as shown. A boiler at one end is used to heat the
substance with which it is desired to impregnate the wood. An air pump
is also employed to remove the steam,