AIR-Shafts, among Miners, denote holes or shafts let down from the open air to meet the adits and furnish fresh
air. The damps, deficiency, and impurity of air which occur, when adits are wrought 30 or 40 fathoms long, make it necessary to let down air-shafts, in order to give the air liberty to play through the whole work, and thus discharge bad vapours, and furnish good air for respiration: the expense of which shafts, in regard of their vast depths, hardness of the rock, drawing of water, &c. sometimes equals, nay exceeds, the ordinary charge of the whole adit.
Air-Threads, in Natural History, a name given to the long filaments so frequently seen in autumn floating about in the air.
These threads are the work of spiders, especially of that species called the long-legged field spider, which, having mounted to the summit of a bush or tree, darts from its tail several of these threads, till one is produced capable of supporting the creature in the air. On this it mounts in quest of prey, and frequently rises to a very considerable height.
Air-Trunk, the name of a contrivance by Dr Hales, to prevent the stagnation of putrid effluvia in jails and other places, where a great number of people are crowded together in a small space. It consists only of a long square trunk open at both ends, one of which is inserted into the ceiling of the room, the air of which is required to be kept pure, and the other extends a good way beyond the roof. Through this trunk a continued circulation is carried on; and the reason is, that the putrid effluvia which do so much mischief when collected, being much lighter than the pure atmosphere, arise to the top of the room, and, if they there find a vent, will continually go out through it. These effluvia arise in very considerable quantity, being calculated by Dr Keil at no less than 39 ounces from one man in 24 hours.
Air-Vessels are spiral ducts in the leaves, &c. of plants, supposed to be analogous to the lungs of animals, in supplying the different parts of a plant with air.