lbs. The engine itself, from 90 to 100 Weight of inflamed coals in a } cavity presenting about 4 feet } 25 surface of tube } Supply of coal for 1 hour 6 Water for ditto, allowing steam } of one atmosphere to be 1/1800 } 32 the specific gravity of water } --- 163 --- It may at first view appear superfluous to inquire further relative to a first mover for aërial navigation; but lightness is of so much value in this instance, that it is proper to notice the probability that exists of using the expansion of air by the sudden combustion of inflammable powders or fluids with great advantage. The French have experimentally shown the great power produced by igniting inflammable fluids in close vessels; and several years ago, an engine was made in this country to work in a similar manner, by the inflammation of spirit of tar. It appears that eighty drops of this fluid raised eight hundred weight to the height of 22 inches; hence a one-horse power may consume from 10 to 12 pounds per hour, and the engine itself need not exceed 50 pounds weight. Probably a much cheaper engine of this sort might be produced by gas-light apparatus, and by firing the inflammable air generated, with a due portion of common air, under a piston. Upon some of these principles it is perfectly clear that force can be obtained by a much lighter apparatus than the muscles of animals or birds, and therefore in such proportion may aërial vehicles be loaded with inactive matter. Even the high pressure steam-engine doing the work of six men, and only weighing equal to one, will readily raise five men into the air, but by increasing the magnitude of the engine ten, fifty; or even five hundred men may equally well be conveyed. Having rendered the accomplishment of this object probable upon the general view of the subject, it will now be necessary to point out the principles of the art itself. The whole problem is confined within these limits, viz. To make a surface support a given weight by the application of power to the resistance of the surrounding atmosphere. Many experiments have been made upon the direct resistance of air by Mr. Robins, Mr. Rouse, Mr. Edgeworth, Mr. Smeaton, and others. The result of Mr. Smeaton's experiments and observations was, that a surface of one square foot met with a resistance of one pound, when it travelled perpendicularly to itself through air at a velocity of 21 feet per second.