376

Air Machines.

[Book IV.

other permanently elastic fluids, m a geometrical progression to equal in-crements of heat. A volume of air at ordinary temperatures is increasedover one third, if raised to 212° Fahrenheit. At the fusing point of lead(about 600°) it is more than doubled, and at the heat of 1100°, it wouldbe tripled. Let a small glass tube be attached to the neck of a Florenceflask, and heat both in boiling water ; if the end of the tube be then placedin mercury, the latter will as the air becomes cooled rise in the tube, to theheight of ten inches—equal to about eleven feet of water. If they wereheated to 600° it would rise to fifteen inches ; and if to 1100° to twentv-one or two inches. We have connected a tube to the mouth of a commonquart bottle, and after heating the latter over a fire, placed the end ofthe tube in mercury, and on remoying the whole to the open air, then at50°, the merSury in a few minutes rose to sixteen inches; hence rathermore than one half of the air had been expelled by the heat. Theseeffects take place when the enclosed air is dry ; but if it be moist, or if adrop or two öf water be in the vessel, the results are greater, because thevapor of the liquid would alone fill or nearly fill the vessel and woulddrive out a corresponding quantity of air.

This mode of creating a vacuum and raising water by the dilatation andcondensation of air is now seldom used, because superior results, as justintimated, are obtained from steam and with less expense. Air machinesare however interesting in several respects. They are among the earliestexamples of elastic fluids being employed as a moving power, induced byalternate changes of temperature. They constitute the first link in thatchain of devices that has now terminated in the steam engine, but whichwill probably be prolonged through future ages by the addition of evenmore efficient mechanism. In this view of the subject, air machines willconnect the researches and inventions of antiquity, in the development andapplications of the most valuable because most pliable of all motive forces,with every improvernent future engineers may make to the end of time.

The oldest air-machines known were made in Egypt , and the oldestaccount extant of such devices is also derived from that country, viz. fromthe Spiritalia. It is also worthy of remark that they are associated byHeron with other devices of the priesthood, for exciting wonder and per-forming prodigies before the people ; thus alfording a collateral proof thatoccupants of the ancient temples at Thebes , Memphis and Heliopoliswere intimately acquainted with the principles of natural philosophy;and fully capable of teaching those who flocked to them for Information,from Greece and neighboring countries. There is a circumstance toothat indicates a more thorough and practical acquaintance with the me-chanical properties of elastic fluids, and the means of exciting those pro-perties than might at first be supposed, viz. in the Substitution of thesun’s heat for that of ordinary fires. This seems to have been adopted incases where the miracle to be wrought could not be accomplished by thelatter without danger of detection, or when it could not be so secretlyeffected, or could not be performed with such imposing effect. Ofthis, the vocal Statue on the plain of Thebes is an example . This gi-gantic idol saluted the rising sun and continued to utter sounds as long asthe solar beams were shed over it, and while surrounded by the myriadsthat worshipped at its shrine. Now these sounds were produced, accor-ding to Heron , by the dilatation of air, or by vapor evolved by the sun’sheat from water contained in close vessels, that were concealed in or con-nected to the base of the Statue , and exposed to the solar rays. Theexpanded fluid, it is supposed, was conveyed through tubes whose orificeswere fashioned to produce the required sounds.