First, those which draw their supplies directly from the atmosphere, and discharge them into the atmosphere again after they have produced their effect.
Such are the Ericsson, Stillman, Roper, Baldwin, Messer, Wilcox engines, described on pp. 40-43. See also Dr. Barnard's report on the French Exposition, pp. 34-40, and plate 1.
Second, those which employ continually the same air, which is alternately heated and cooled, but which is not allowed to escape.
Such are the Glazebrook (1797), Parkinson and Crosley (1827), Laubereau (1849), Schwartz, described on pp. 43, 44.
These and other distinguishing features are described under air-engine(which see).
One in which air is heated for warming houses, or for purposes of drying, usually the former.
The arrangements are various, but a common kind is a form of stove in a brick chamber, the air coursing around the stove and among certain pipes and chambers in which circulate the volatile products of combu