THE AIR-PUMP AND THE AIR-GUN. IMMEDIATELY after the discovery of the principle of the Barometer by Torricelli, in the pressure of the air on the general surface, followed that of Otto von Guericke, whose aim seems to have been to decide the question, whether a vacuum could or could not exist, by endeavouring to make one.* The first Air-pump constructed by Guericke was exhibited by him at the Imperial Diet of Ratisbon in 1654. It was an exhausting syringe, attached underneath a spherical glass-receiver, and worked somewhat like a common pump. The syringe was entirely immersed in water, to render it air-tight. The imperfection of his mechanism, however, enabled Guericke only to diminish the aerial contents of his receiver, not entirely to empty them; but the curious effects produced by even a partial exhaustion of air speedily excited attention, and induced our illustrious countryman, Robert Boyle, to construct an air-pump, in which the syringe was so far improved that the water could be dispensed with: he also first applied rack-work to the syringe. In the Journals of the Royal Society, Jan. 2d, 1660, we find Boyle's air-pump referred to as his Cylinder, and "that Mr. Boyle be desired to show his Experiments of the Air," which are printed in the Society's Transactions. The Air-pump constructed by Boyle was presented to the Society by him in 1662, and it is now in the museum at Burlington House: the pump consists of two barrels. We have the testimony of a French savant of the nineteenth century, M. Sibes, that the Air-pump in Boyle's hands became a new machine; and Professor Baden Powell considers that "he reduced it nearly to its present construction." It is true that the second syringe and the barometer gauge were afterwards added by Hawksbee, and several minor improvements *This ingenious and ardent cultivator of science, who was born at Magdeburg, in Saxony, in the beginning of the seventeenth century, in his original attempts to produce a vacuum, used first to fill his vessel with water, which he then sucked out by a common pump, taking care, of course, that no air entered to replace the liquid. It was by first filling it with water that Guericke expelled the air from the copper globe, the two closely fitting hemispheres comprising which six horses were then unable to pull asunder, although held together by nothing more than the pressure of the external atmosphere. This curious proof of the force or weight of the air, which was exhibited before the Emperor Ferdinand III., in 1634, is commonly referred to by the name of the experiment of the Magdeburg Hemispheres. Guericke, however, afterwards adopted the method of exhausting a vessel of its contained air by the air-pump. Effects of the Air-pump. 31 were made by Hooke, Mariotte, Gravesande, and Smeaton. All the alterations which have been made since the time of the invention, however important, relate to the mechanism only, and not to the principle on which the pump acts. Dr. Hutton has grouped these effects and phenomena of the Air-pump. In the exhausted receiver, heavy and light bodies fall equally swiftly so, a guinea and a feather fall from the top of a tall receiver to the bottom exactly together. Most animals die in a minute or two: however, vipers and frogs, although they swell much, live an hour or two, and after being seemingly quite dead, revive in the open air. Snails survive about ten hours; efts, two or three days; leeches, five or six. Oysters live for twenty-four hours. The heart of an eel, taken out of the body, continues to beat for great part of an hour, and that more briskly than in the air. Warm blood, milk, gall, &c., undergo a considerable internescence and ebullition. Eggs of silkworms hatch in vacuo. Vegetation stops. Fire is extinguished; the flame of a candle usually going out in one minute, and charcoal in about five minutes. Red-hot iron seems, however, not to be affected; sulphur and gunpowder are not lighted by it, only fused. A match, after lying seemingly extinct for a long while, revives on re-admitting the air. A flint and steel strike sparks of fire as copiously as in air. Magnets and magnetised needles act as in air. Heat may be produced by attrition. Camphor will not take fire; and gunpowder, though some of the grains of a heap of it be kindled by a burning-glass, will not give fire to the contiguous grains. Glow-worms lose their light in proportion as the air is exhausted; but on re-admitting the air they presently recover. A bell, on being struck, is not heard to ring, or very faintly. Water freezes. A syphon will not run; and electricity appears like the Aurora Borealis. 66 66 De la Croix relates the following instance of sagacity in a cat, who, even under the receiver of an Air-pump, discovered the means of escaping a death which appeared to all present inevitable. "I once saw," he relates, a lecturer upon experimental philosophy place a cat under the glass-receiver of an Air-pump, for the purpose of demonstrating that life cannot be supported without air and respiration. The lecturer had already made several strokes with the piston, in order to exhaust the receiver of its air, when the cat, who began to feel herself very uncomfortable in the rarefied atmosphere, was fortunate enough to discover the source from whence her uneasiness proceeded. She placed her paw upon the hole through which the air escaped, and thus prevented any more from passing out of the receiver. All the exertions of the philosopher were now unavailing in vain he drew the piston; the cat's paw effectually prevented its operation. Hoping to effect his purpose, 32 Theory of the Air-gun. he again let air into the receiver, which, as soon as the cat perceived, she withdrew her paw from the aperture; but whenever he attempted to exhaust the receiver, she applied her paw as before. The spectators clapped their hands in admiration of the cat's sagacity; and the lecturer was compelled to remove her, and substitute another cat that possessed less penetration for the cruel experiment." Although the Air-pump is scarcely two centuries old, yet the Air-gun, which is so nearly allied to it in the construction of its valve and condensing syringe, existed long antecedent to it; for it is recorded that an Air-gun was made for Henry IV., by Marim, of Lisseau, in Normandy, as early as 1408; and another was preserved in the armory at Schmetau, bearing the date of 1474. The Air-gun of the present day is different. Bishop Wilkins mentions "the Wind Gun" as a late ingenious invention, which discharges with force "almost equal to our powder guns." Professor Helmholtz, one of the latest illustrators of this instrument, thus lucidly explains its theory: "Into the chamber of an Air-gun we squeeze, by means of a condensing air-pump, a great quantity of air. When we afterwards open the cock of the gun, and admit the compressed air into the barrel, the ball is driven out of the latter with a force similar to that exerted by ignited powder. Now we may determine the work consumed in the pumping-in of the air, and the living force which, upon firing, is communicated to the ball; but we shall never find the latter greater than the former. The compressed air has generated no working force, but simply gives to the bullet that which has been previously communicated to it. And while we have pumped for perhaps a quarter of an hour to charge the gun, the force is expended in a few seconds when the bullet is discharged; but because the action is compressed into so short a time, a much greater velocity is imparted to the ball than would be possible to communicate to it by the unaided effort of the arm in throwing it." We may here relate a curious wager, which Sir Robert Moray, at the request of Charles II., brought forward at a meeting of the Royal Society in 1671. It was-that the king wagered 50l. to 5l. " for the compression of air by water." It was accordingly resolved that Mr. Hooke should prepare the necessary apparatus for the experiment, which Sir Robert Moray said "might be done by a cane, so contrived that it should take in more and more water, according as it should be sunk deeper and deeper into it." The minutes of a subsequent meeting record the successful performance of the experiment; and that it 66 was acknowledged his majesty had won the wager." LIVING UNDER WATER : THE DIVING-BELL. WHEN We consider the vast amount of treasure which has been from time to time lost in the depths of the sea, we shall not be surprised at the variety of the means which have been devised for the recovery of the hidden wealth. The principal of these contrivances is the Diving-bell, with the operations of which the public have become familiar by the exhibition of an improved bell at our Polytechnic Institution;* but the history of the invention, as well as the primitive means by which it was preceded, present many interesting instances of ingenuity directed to humane and praiseworthy purposes. In remote ages (says Professor Beckmann), divers were kept in ships to assist in raising anchors, and goods thrown overboard in times of danger; and, by the laws of the Rhodians, they were allowed a share of the wreck proportioned to the depth in which they had gone in search of it. In war, they were often employed to destroy the works and ships of the enemy; divers also fished for pearls. The statements of their remaining under water unassisted by apparatus for procuring air are, however, greatly exaggerated; they speak of six hours, whereas six minutes is the longest time of submersion recorded in modern times. Dr. Halley, in a paper in the Philosophical Transactions, on "the Art of Living under Water," describes the divers for *For twenty years (1839-1859), there was exhibited at the Polytechnic Institution, No. 300 Regent Street, London, a diving-bell, which was put in operation daily. This bell was manufactured by Cottam and Hallen, and cost about 4007. It is of cast-iron, and weighs 3 tons; 5 feet in height, and 4 feet 8 inches in diameter at the mouth. Within is affixed a knocker, under which is painted: "More air, knock once; Less air, knock twice; The bell is about one-third open at the bottom, has a seat all round for the divers, is lit by twelve openings of thick plate-glass. It is suspended by a massive chain to a large swing-crane, with a powerful crab; the chain having compensation-weights, and working into a well beneath. The air was supplied from two powerful air-pumps, of eight-inch cylinder, conveyed by the leather hose to any depth: the divers being seated in the bell, it was moved over the water, and directly let down within two feet of the bottom of the tank, and then drawn up; the whole occupying only two minutes and a half. The tank and the adjoining canals held 10,000 gallons of water. Each person descending in the bell paid 1s.; and it has produced 10007. in one year. Ꭰ 34 Living under Water. sponges in the Archipelago taking down in their mouths a piece of sponge soaked in oil, by which they were enabled to dive for a longer period than without it. As the bulk of the sponge must diminish the quantity of air which the diver could contain in his mouth, it does not appear probable that this practice could assist respiration. In connection with Diving by the unassisted powers of the body, Professor Faraday relates this curious fact: The lungs are, in their natural state, charged with a large quantity of impure air; this being a portion of the carbonic-acid gas which is formed during respiration, but which, after such expiration, remains lodged in the involved passages of the pulmonary vessels. By breathing hard for a short time, as a person does after violent exercise, this impure air is expelled, and its place is supplied by pure atmospheric air, by which a person will be enabled to hold his breath much less longer than without such precaution. Dr. Faraday states that, although he could only hold his breath, after breathing in the ordinary way, for about three-quarters of a minute, and that with great difficulty, he felt no inconvenience, after making eight or ten forced respirations to clear the lungs, until the mouth and nostrils had been closed more than a minute and a half; and that he continued to hold breath to the end of the second minute. A knowledge of this fact may enable a diver to remain under water at least twice as long as he otherwise could do. Possibly the exertion of swimming may have the effect of clearing the lungs, so that persons accustomed to diving may unconsciously avail themselves of this preparatory measure. The advantage of breathing condensed air, and thereby obtaining a larger supply of oxygen in the same bulk than with air of the ordinary pressure, is shown also in the following fact: After one of the disastrous occurrences at the works of the Thames Tunnel, Mr. Brunel, the engineer, descended in a divingbell to examine the breach made by the irruption of the river into the tunnel. The bell was lowered to the mouth of the opening, a depth of about thirty feet; but the breach was too narrow to allow it to go lower, in order that the shield and other works, which lay eight or ten feet deeper, might be examined from the bell. Mr. Brunel, therefore, took hold of the rope, and dived below the bell for the purpose. After he had remained under water about two minutes, his companion in the bell became alarmed, and gave a signal which caused Brunel to rise. On doing so, he was surprised to find how much time had elapsed; and, on repeating the experiment, he ascertained that he could with ease remain fully two minutes under water; a circumstance accounted for by the condensation of the air in the bell, from which his lungs were supplied by the pressure of a co |