Air Beds.

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Chap. l.J

incumbent strata, and thus at a certain height this small globule of airwould occupy a space equal to tbe eartb ltself! And at the height of fouror five hundred miles, it has been calculated, that less than a teacup fullof the air we breathe, would fill a sphere equal in diameter to the veryorbit of Saturn ! The efficiency of the air pump in producing a vacuumdepends entirely on this property.

Air at the foot of a mountain, whose elevation is between three andfour miles, occupies twice the space when carried to tbe top. A quart ofit taken from the summit would be reduced to a pint if conveyed to thebottom. From this expansive power of air arises its elasticity. This isfamiliär to most people ; for when confined in flexible vessels, as air beds,pillows, life preservers, &c. as soon as any weight or pressure imping-ing upon themis removed, the elasticity of the confined fluid pushes upthe depressed part as before. If air within a bladder were not elastic, theimpressions made by the fingers in handling it would remain as in a ballof paste, and air beds would retain the form of bodies that reposed uponthem, like a founder’s mould of sand or plaster.® Those extremely light

a Air beds are not, as some persons suppose, of modern origin. They were knownbetween three and four hundred years ago, as appears from the annexedeut, (No. 68,)copied from some figures attached to the first German translation of Vegetius , A. D.1511. It represents soldiers reposing onthem in time of war, with the mode of infla-ting them by bellows.

No. 68. Ancient Air Bed.

This application- of air was probably known to the Romans. Heliogabalus used toamuse himself with the guests he invited to his banquets, by seating them on large bagsgrou d " wind ’” whic * 1 being made suddenly to collapse, threw the guests on the

Dr-Amott, the author of * Elements of Physies,’ a few years ago, proposed ‘ Hydro-static beds,’ especially for invalids. These are capacious bags, formed of india-rubberc oth, and filled with water instead of feathers, hair, &c. Upon one of these a soft and thinmattress is laid, and then the ordinary coverings. A person floats on these beds as on

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