The plate with the glass attached to it is inverted, and compressed air or steam is introduced through openings in the plate.
By using compressed air vitiation of the mine air is avoided, as well as all danger of fire or explosion of gas.
In mines that are worked from the outcrop by adits or day levels traction by locomotives driven by steam, compressed air or electricity is used to some extent.
For heavy gradients rope haulage has no rival, though for moderate grades it is often advantageously replaced by electric and compressed air haulage.
Anaximenes, a pupil of Anaximander, was the first to reject the view that the earth was a circular plane, but held it to be an oblong rectangle, buoyed up in the midst of the heavens by the compressed air upon which it rested.
The first difficulty was to make it sufficiently light in relation to the power its machinery could develop; and several machines were built in which trials were made of steam, and of compressed air and carbonic acid gas as motive agents.
In such compressed-air machines as are now made there is no injection of water during compression, and the compressed air is cooled in a surface cooler, not by actual mixture with a shower of cold water.
Trolley haulage lacks the flexibility of steam or compressed air haulage, and is limited to main lines because the wires must be strung throughout the length of the line.
Steam locomotives have been largely superseded by compressed air or electric locomotives.
The use of small auxiliary blowing ventilators underground, for carrying air into workings away from the main circuits, which was largely advocated at one time, has lost its popularity, but a useful substitute has been found in the induced draught produced by jets of compressed air or high-pressure water blowing into ejectors.