and it is also possible for mine inspectors and fire bosses to bring
their safety lamps to this station, and test their measurements of
percentage of gas, by noting the length and the appearance of the flame
in the presence of mixtures containing known percentages of methane and
air.

[Illustration: PLATE XI.

Fig. 1.--Impact Machine.

Fig. 2.--Lamp Testing Box.]

The gas-tight gallery used for testing the lamps, consists of a
rectangular conduit (Fig. 2, Plate X), having sheet-steel sides, 6 mm.
thick and 433 mm. wide, the top and bottom being of channel iron. The
gallery rests on two steel trestles, and to one end is attached a No. 5
Koerting exhauster, capable of aspirating 50 cu. m. per min., under a
pressure of 500 mm. of water, with the necessary valve, steam separator,
etc. The mouth of the exhauster passes through the wall of the building
and discharges into the open air.

Besides the main horizontal conduit, there are two secondary conduits
connected by a short horizontal length, and the whole is put together so
that the safety lamp under test may be placed in a current of air, or of
air and gas, which strikes it horizontally, vertically upward or
downward, or at an angle of 45° (Fig. 3). The path of the current is
determined by detachable sheet-steel doors.

[Illustration: Fig. 3.