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BOOK II.

MACHINES FOR RAISING WATER BY THE PRESSURE OF THE
ATMOSPHERE.

CHAPTER I.

ON machines that raise water by atmospheric pressure-Principle of their action formerly unknownSuction a chimera-Ascent of water in pumps incomprehensible without a knowledge of atmospheric pressure-Phenomena in the organization, habits, and motions of animals-Rotation of the atmosphere with the earth-Air tangible-Compressible-Expansible-Elastic-Air beds-Ancient beds and bedsteads-Weight of air-Its pressure-Examples-American Indians and the air pump-Boa Constrictor -Swallowing oysters-Shooting bullets by the rarefaction of air-Boy's sucker-Suspension of flies against gravity-Lizards-Frogs-Walrus-Connection between all departments of knowledge-Sucking fish-Remora-Lampreys-Dampier-Christopher Columbus at St. Domingo-Ferdinand Columbus -Ancient fable-Sudden expansion of air bursting the bladders of fish-Pressure of the atmosphere on liquids.

WITH the last chapter we concluded our remarks on machines embraced in the first general division of the subject, (see page 8) and now proceed to those of the second; viz. such as raise water by means of the weight or pressure of the atmosphere. These form a very interesting class they are genuine philosophical instruments, and as such may serve to exhibit and illustrate some of the most important truths of natural philosophy. The principle upon which their action depends was formerly unknown, and even now, a person, however ingenious, while ignorant of the nature and properties of the atmosphere, would be utterly unable to account for the ascent of water in them. Having no idea of the cause of this ascent, except the vague one of suction, he would feel greatly embarrassed if required to explain it. And when informed that there really is no such thing in nature as suction, but that it is a mere chimera, having no existence except in the imagination, the task would be attended with insuperable difficulties. Perhaps he would have recourse to a common pump, to trace, if possible, the operation in detail; if so, he would naturally begin with the first mover, or the pump handle, and would look for some medium, by which motion is transmitted from it, to the water in the well; but, however close the scrutiny might be made, he would be unable to detect any; and as a matter of course, while a connection between them, i. e. between the mover and the object moved, could not be discovered, it would be impossible for him satisfactorily to account for the phenomenon. If "a body cannot act where it is not present," as the sucker of a pump, on water at a distance from it, how could such a person account for the ascent of that water in obedience to the movements of the sucker? And how could he explain the process by which it was effected,

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174

Ascent of Water in Pumps incomprehensible

[Book II. while he could find no apparent communication between them? The fact is it would be difficult for him to point out any closer connection between the pump rod and the water in the well, than between a walking cane in the hands of a pedestrian, and water under the surface of the ground over which he stepped; nor could he assign a conclusive reason, why the liquid should not ascend and accompany the movements of the latter as well as of the former.

He could perceive no obvious or adequate cause for the elevation of water through the pipe of a pump, there being no apparent force applied to it, or in the direction of its ascent, no vessel or moveable pallet going down, as in the preceding machines, to convey or urge the liquid up-and hence he could no more comprehend how the movements of a pump box (sucker) above the surface of the ground, should induce water in a well to rush up towards it, than he could explain how the waving of a magician's wand should cause spirits to appear.

Long familiarity with the atmospheric pump, makes it hard for us, at the present day, to realize the difficulties formerly experienced in accounting for the ascent of water in it. Suppose the cause yet unknown and unthought of it certainly would puzzle us to explain how a piece of leather (the sucker) moving up and down in a vertical tube, whose lower orifice is in water, some twenty-five or thirty feet below it, should conjure that water up. Such a result is opposed to all experience and observation in other departments of the arts; nor is there any thing like it, in the machines we have examined in the preceding book. The mechanism by which motion is transmitted from them to the water, is obvious to the senses a tangible medium of communication is established between the force that works them and the water they raise; whereas in the pump, an invisible agent is excited, whose effects are as surprising as its mode of operation is obscure. 'Tis true, a tube (the pump pipe) is continued from the place where the sucker moves to the water, but it remains at rest, or is immoveable, and therefore cannot transmit motion from one to the other; it is merely a channel through which the water may rise-it does not raise it.

But if, in order to establish a connection between the sucker and water, the former were made to descend through the pump into the latter, still the difficulty would not be overcome. The sucker in that case would act much like one of those buckets, used in some wells, which has an opening in its bottom to admit the water, and covered by a flap to prevent its return. (The sucker is in fact, merely a small bucket of this kind, and is so named in some countries.) In both cases the water would be raised which entered through the valves-the bucket would bring up all it contained, and the sucker all that passed through it into the pump; so far the operation of both is clear, and as regards the raising of the water above the valves, would be the same; but it is the ascent of a column of water behind the sucker that requires explanation-a liquid column that follows it as closely through every turn of the tube, as if it were a rope, having its fibres at one end fastened to the sucker and pulled up by it. What is it that makes this water ascend against a law of its nature-against gravity? Were the cohesion of its particles such that it could be raised by a force applied only to its upper end, then indeed the difficulty would be diminished; but in that case, it would follow that a similar column would ascend after a bucket when drawn out of an open well; and further, that a traveler might then make use of a liquid walking stick, to assist him in his journeying.

Baffled thus in our attempts to find a solution here, we perhaps would

Chap. 1.]

Without a knowledge of Atmospheric Pressure.

175

begin to think, that when a liquid is raised in the pipe of a pump, it must be by some force acting below, or behind it, a force a tergo, as it is named, and of which all the preceding machines are examples. Thus when a bucket of water is raised from a well, the force is applied behind it, i.e. to the bottom of the bucket, through the cord, bale, and sides, to which it is attached. It is the same in the screw, the force continually elevating a portion of it immediately behind the water; and in the tympanum, noria, chain of pots, chain pump, &c. it is the same; the vessels or pallets go below, i. e. behind the liquid and urge it up before them. It is the same in all ordinary motions. I wish to examine a small object laying at the foot of my garden: now I cannot by moving this ruler in its direction, in the manner of a pump rod, induce it to move to me, nor can it ever be so moved, until the force of some other body in motion behind it impel it towards me. It is the same in the case of a stubborn boy, who not only refuses to move as directed, but opposes the natural inertia of his body to the change, and therefore can only be impelled forward by some force applied directly or indirectly behind him, by dragging or pushing him along. In this way, all the motions in the universe, according to some philosophers, are imparted or transferred; those which appear exceptions being considered modifications of it. Still however, the difficulty of establishing a connection between the movements of the sucker in the interior of the pump at one end, and this force, whatever it might be, acting on the water, outside of it, at the opposite end, would remain; and we should probably at last impute this ascent of water (with the ancients) to some indefinable energy of nature, both fallacious and absurd; nor would this be surprising, for in the absence of a knowledge of the atmosphere and of its properties, there really is as great a MYSTERY in the movements of a pump rod being followed by the ascent of the liquid, as in any thing ever attributed to the divining rod, or to the wand of Abaris.

In order to understand the operation of machines belonging to this part of the subject, and also the principle upon which their action depends, we must leave, for a few moments, the consideration of pumps and pipes, and all the contrivances of man, and turn our attention to some of the Creator's works as they are exhibited in nature. This may perhaps be deemed a departure from the subject; it is however so far from being a digression, that it is essentially necessary to ascertain the cause of water ascending in this class of machines, as well as to understand the philosophy of numerous natural as well as artificial operations, that are performed by apparatus analogous to them; as the acts of inspiration and respiration, quadrupeds drinking, the young of animals sucking their dams, children drawing nourishment from their mothers' breasts: bleeding by cupping, by leeches, or by the more delicate apparatus of a musketoe's proboscis; and if things ignoble may be named, the taking of snuff, sinoking of cigars and pipes of tobacco, and also the experiments of those peripatetic philosophers, who perambulate our wharves, and imbibe nectar through straws from hogsheads of rum and molasses.

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Every person is aware, that the earth on which we live is of a globular or spheroidal figure, and that it is enveloped in an invisible ocean of air or gas, which extends for a great number of miles from every part of its surface. This hollow sphere of air is named the atmosphere, and is one of the most essential parts in the economy of nature. It is the source as well as the theatre of those sublime meteorological phenomena which we constantly behold and admire. It is necessary to animal and to vegetable life. Its material is the breath of life' to all things living. It is more

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over the peculiar element of land animals, the scene of their actions, the

176

Compressibility and Dilatability of Air.

'Book II. fluid ocean in which they only can move, and within which they are always immersed. It is to them, what the sea is to fish: remove them from it, and they necessarily die. In some respects nature has been more favorable to fishes than to us: most of them can ascend to the surface of the fluid in which they live, but we can only exist in the lowest depths of the atmospheric ocean that confines us: if we ascend but a little, our energies begin to fail, and we are compelled to descend to the bottom, the place she designed us to occupy.

Possibly, some people may suppose that the velocity with which the earth shoots forward in her orbit, might sometimes cause this atmosphere (which hangs as a mantle so loosely about her) to be left floating, like the tail of a comet, behind; or be entirely separated from her, like the cloud of vapor which the impetuous ball leaves at the cannon's mouth. Such however is not the fact; on the contrary, it revolves uniformly with the earth on the axis of the latter, and accompanies her, as a part of herself, round the sun. Were it indeed separated from her, but for a moment, either by an increase or diminution of her velocity, the present organization of nature would be destroyed; every mountain would be hurled from its base; every house on the globe would be leveled; and no human being could survive. Had the atmosphere not a rotatory motion also, in common with that of the earth, i. e. of the same velocity and in the same direction, a very different state of things, as regards the arts, would have subsisted than those which we behold. For example, aerial navigation would certainly have superseded nearly all traveling by land and water; and railroads, and locomotive carriages, and steamboats, would hardly have been known; for the project of that individual who proposed to visit distant countries, by merely ascending in a balloon, till the rotation of the earth on its axis brought them under him, when he intended to descend, would have been no visionary scheme.

The air is tangible.-Although the substance of the atmosphere is not visible, it is tangible; we feel it when in motion as wind, whether it be gently disturbed as in the evening breeze, or by the slight waving of a lady's fan; or when greatly excited, as in the hurricane, or the violent blast from a bellows' mouth. We also see its effects when thus in motion, in the direction of smoke, extinction of our tapers, slamming of doors, in the beautiful waving of grass, and of the full eared grain of the fields; trees yielding to its impulse, buildings unroofed, and sometimes in the prostration of large tracts of forests; in windmills, sailing of ships, and the convulsions into which it throws the otherwise placid ocean.

Air is compressible.-Indeed compressibility and expansibility are properties of all bodies; by the abstraction of heat, airs are compressed into liquids, and liquids into solids, while an increase of temperature expands solids into liquids, and these into airs. In the common air gun, four or five gallons of the dense air around us are compressed into a pint, and by further pressure they may be squeezed into a few drops of liquid, which a tea spoon might contain.

Its expansibility or dilatability is, so far as known, illimitable; the space it occupies being always in proportion to the pressure that confines it. If a collapsed and apparently empty bladder be placed under a receiver, and the air around its exterior be removed, the small portion within will expand and swell it out to its natural shape. If it were possible to withdraw the whole of the air from this room, and a globule no larger than a pea were then admitted, it would instantly dilate and fill the room. The upper strata of the atmosphere decrease in density as they recede from the earth's surface, on account of the diminution of the pressure from

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Chap. 1.]

Air Beds.

177

incumbent strata, and thus at a certain height this small globule of air would occupy a space equal to the earth itself! And at the height of four or five hundred miles, it has been calculated, that less than a teacup full of the air we breathe, would fill a sphere equal in diameter to the very orbit of Saturn! The efficiency of the air pump in producing a vacuum depends entirely on this property.

Air at the foot of a mountain, whose elevation is between three and four miles, occupies twice the space when carried to the top. A quart of it taken from the summit would be reduced to a pint if conveyed to the bottom. From this expansive power of air arises its elasticity. This is familiar to most people; for when confined in flexible vessels, as air beds, pillows, life preservers, &c. as soon as any weight or pressure impinging upon them is removed, the elasticity of the confined fluid pushes up the depressed part as before. If air within a bladder were not elastic, the impressions made by the fingers in handling it would remain as in a ball of paste, and air beds would retain the form of bodies that reposed upon them, 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 known between three and four hundred years ago, as appears from the annexed cut, (No. 68,) copied from some figures attached to the first German translation of Vegetius, A. D. 1511. It represents soldiers reposing on them in time of war, with the mode of inflating them by bellows.

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This application of air was probably known to the Romans. Heliogabalus used to amuse himself with the guests he invited to his banquets, by seating them on large bags or beds, "full of wind," which being made suddenly to collapse, threw the guests on the ground.

Dr. Arnott, the author of Elements of Physics,' a few years ago, proposed Hydrostatic beds,' especially for invalids. These are capacious bags, formed of india-rubber cloth, and filled with water instead of feathers, hair, &c. Upon one of these a soft and thin mattress is laid, and then the ordinary coverings. A person floats on these beds as on

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