le wooden shutter
sliding in a groove in a cast iron plate, curved to the slope of the
casing. By the use of the spiral guide casing and the chimney the
velocity of the effluent air is gradually reduced up to the point of
final discharge into the atmosphere, whereby a greater useful effect is
realized than is the case when the air streams freely from the
circumference with a velocity equal to that of the rotating fan. The
power is applied by steam acting directly on a crank at one end of the
axle, and the diameter of the fan may be 40 ft. or more.

[Illustration: FIG. 12.--Guibal Fan.]

The Waddle fan, represented in fig. 13, is an example of another class
of centrifugal ventilator, in which a close casing is not used, the air
exhausted being discharged from the circumference directly into the
atmosphere. It consists of a hollow sheet iron drum formed by two
conoidal tubes, united together by numerous guide blades, dividing it up
into a series of rectangular tubes of diminishing section, attached to a
horizontal axle by cast iron bosses and wrought iron arms. The tubes at
their smallest part are connected to a cast iron ring, 10 ft. in
diameter, but at their outer circumference they are only 2 ft. apart.
The extreme diameter is 25 ft.

[Illustration: FIG. 13.--Waddle Fan.]

By the adoption of more refined methods of construction, especially in
the shape of the intake and discharge passages for the air and the forms
of the fan blades, the efficiency of the ventilating fan has been
greatly increased so that the dimensions can be much reduced and a
higher rate of speed adopted. Notable examples are found in the Rateau,
Ser and Capell fans, and where an electric generating station is
available electric motors can be advantageously used instead of steam.

Distribution of air underground.

The quantity of air required for a large colliery depends upon the
number of men employed, as for actual respiration from 100 to 200 cub.
ft. per minute should be allowed. In fiery mines, however, a very much
larger amount must be provided in order to dilute the gas to the point
of safety. Even with the best arrangements a dangerous increase in the
amount of gas is not infrequent from the sudden release of stored-up
masses in the coal, which, overpowering the ventilation, produce
magazines of explosive material ready for ignition when brought in
contact with the flame of a lamp or the blast of a shot. The management
of such place