AIR-PRESSURE ENGINES. Engines in which the difference of pres sure of air of different densities is employed as a motive force. From the extreme lightness and mobility of air, it has been frequently proposed to employ it as a medium for transmitting motion to machinery at a considerable distance from the prime mover. Amongst the first who attempted this, is the cele brated Pepin, who invented the steelyard safety-valve. He employed a fall of water to compress the air in a cylinder, through the medium of an interven ing piston; and he connected this cylinder to another, at the mouth of a mine a mile distant, by means of a pipe of that length. In the second cylinder was another piston, the rod of which was intended to work a set of pumps ; but contrary to expectation, the compression of the air in the first cylinder produced no movement in the piston of the second. Pepin subsequently attempted to bring his scheme into use in England, but did not succeed. Afterwards, how ever, he erected great machines in Auvergne and Westphalia, for draining mines, but so far from being effective machines, they would not even begin to move. He attributed the failure to the quantity of air in the pipe, which must be condensed before it can condense the air in the remote cylinder ; he there fore diminished the size of this pipe, and made his water machine exhaust instead of condense, and had no doubt that the immense velocity with which air rushes into a void, would make a rapid and effectual communication of power. But the machine stood still as before. Near a century after this, an engineer at an iron foundry in Wales, erected a machine at a powerful fall of water, which worked a set of cylinder bellows, the blow-pipe of which was con ducted to the distance of a mile and a half, where it was applied to a blast furnace ; but notwithstanding every care to make the conducting pipe very air tight, of great size, and as smooth as possible, it would hardly blow out a candle. The failure was ascribed to the impossibility of making the pipe air-tight ; but above ten minutes elapsed after the action of the piston in the bellows, before the least wind could be perceived at the end of the pipe, whereas the engineer had calculated that the interval would not exceed six seconds. The foregoing particulars are taken ftom Dr. Robinson's Natural Philosophy, art. Pneumatics, and an explanation is, offered of this curious phenomenon ; but on account of its prolixity, we omit it ; but the following remarks, which appeared in the Franklin Journal, are deserving of notice. " If we take a particular care, and calculate the resistance of air moving through pipes according to ac knowledged principles, we shall find nothing mysterious in the above result.
It will be found, that if the blow-pipe is 3 inches in diameter, and only a mile long, the air at one end must be kept constantly condensed by a pres sure equal to 54 atmospheres to produce a velocity of 128 feet per second ; and yet this velocity gives only 2304 gallons per minute, only about half the quantity used in the furnaces of Europe ; a blast furnace there expels 720 cubic feet of air per minute, (Mech. Philos. Vol. VIII. p 784,) if we calculate the velocity of water issuing from a pipe a mile long, and 3 inches in diameter, under a 9-foot head and fall, to be 1 foot per second. Now, as equal velocities are known to be generated in all fluids by equal heads, all other circumstances being equal, it will follow that a 9-foot head of air, or of a head of 9 feet of water, will generate in air a velocity of 1 foot per second in a tube 3 inches in diameter, and a mile long. Again, it is known both from theory and experiment, that the heads of pressure generating velocities in fluids, are as the squares of the velocities ; now the square of 1 is 1, and the square of 128 is 16,384, there fore the head of pressure due to the velocity of 128 feet, is obtained by the following proportion, as 1 : 16384 :: 9 : 147456, and this number divided by 800, gives 184* equal to a pressure of atmospheres, as before said : now if we suppose this velocity doubled, or 256 feet a second, in order to discharge air enough for a blast furnace, the head of pressure must be four times as great, or upwards of 21 atmospheres. This would require a machine of 3426 horse power, provided a horse can work eight hours a day, raising 140 lbs. 200 feet per minute. Notwithstanding the failure of both of the plans of Papin, and the plausible arguments against themjust quoted, they have been recently revived in this country. Mr. Samuel Wright has taken out a patent for trans mitting power to machinery by means of condensed air; but we have not heard of any erections on the principle. Mr. Hague, however, has taken out patents for effecting the same object by the rarefaction of air by an air-pump, and has established several machines upon this principle, the successful operation of which leaves no doubt that the failure of Pepin must have arisen from some defective arrangements, or imperfect workmanship. We have selected a few of those machines to which Mr. Hague considers the principle as peculiarly applicable. The first of these which we shall describe, is a crane for raising goods into lofty warehouses; hut previous to showing the actual arrangement of the machinery, we shall explain the principle by means of the accompanying figure.