This projector can be made at any colliery workshop from a few short pieces of piping and an old oil drum. It may be fixed in the drum or in an open tank of larger capacity placed 12 to 15 yd (11.0 to 13.7 m) back from the face of the hard heading. To the water in the drum, powdered washing soda is added at the ratio of 4 oz (113.4 g) to 5 gal (18.9 L) of water. About 2 min before firing, compressed air is turned on and ejections of 1471 water in the form of a coarse mist fill the heading. This continues for a period of 6 min after the firing. In this manner, the heading is filled with a mist of droplets that outnumber the dust particles; the latter are effectively wetted or become attached to the droplets, with the result that the dust rapidly settles out of the air. This mist projector has a high efficiency, particularly where the ventilation current is low.