water till they reach the top, when they strike against the edge of a fixed reservoir, and being turned over by the stroke, they empty their contents into that reservoir.

Sometimes the Persian wheel is made to raise water only as high as its axle. In this case, instead of buckets, the spokes of the wheel are constructed of a spiral form, and hollow; so that their inner extremities all terminate in a box, at the axle, and their outer extremities in the circumference of the wheel. When, therefore, the rim is immersed in the stream, the water runs into openings in it, and thence through the spiral spokes into the reservoir at the axle, whence it is discharged, or is conveyed in pipes.

The Clepsydra, invented by Ctesibius of Alexandria, was an interesting machine. The water which measured the progress of time by the gradual descent of its surface, flowed in the form of tears from the eyes of a human figure. Its head appeared bent down with age; its look was dejected; while it seemed to pay the last tribute of regret to the fleeting moments as they escaped. The water thus discharged, was collected in a vertical reservoir, where it raised another figure, holding in its hand a rod, which, by its gradual ascent, pointed out the hours upon a vertical column. The same fluid was afterwards employed in the interior of the pedestal, as the impelling power of a piece of mechanism, which made this column revolve round its axis in a year, so that the months and days were always shown by this index, whose extremity de

scribed a vertical line, divided according to the relative lengths of the hours of day and night.

QUESTIONS.

What is the overshot wheel? What is the undershot wheel? What is the breast-wheel? What is the Persian wheel? By whom was invented the machine called the pump? What is the common, or sucking pump? What is the forcing pump? What is the lifting pump? is the chain pump? What is the fire engine? What is the screw engine of Archimedes? What was the clepsydra of Ctesibius of Alexandria?

What

CHAP. XLI. '

PNEUMATICS.

THE Science which contemplates the mechanical properties of all elastic, or sensibly compressible fluids, is termed pneumatics. Of all these fluids, air is the most familiar to human observation, and was, consequently, the first studied, and the most minutely examined.

Air is a material fluid: a fluid because its parts are easily moved; and material, since it has impulsive force; impenetrability, elasticity, inertia, and weight. All these assertions are proved by experiments.

Air is material, since it offers sensible resistance to the hand, or a fan, or any other body moved quickly in it.

Air is elastic; for it can be compressed into a much less space than it usually occupies. When

the compressing power is removed, it regains its former space and situation, with a force equal to the compressing force.

Air has weight or gravity; for if a vessel be emptied of air, it weighs less than when it is full of air. It supports the clouds and vapours. Certain bodies, such as soap bubbles, and other light substances, rise in air, which shows, likewise, that it has weight. Under the name of atmosphere, it surrounds our globe, extending to a considerable distance from it, and accompanying it in its revolution round the sun; which indicates that it is connected with the earth by the general force of gravity, or attraction towards its centre.

Air is a fluid, because its parts are easily displaced.

Air has impulsive force; for when put into violent action, a current of it will carry away very heavy substances.

Air is perfectly transparent; that is, it permits the rays of light to pass through it; and therefore it is invisible.

Air is variable as to its degree of weight. Near the surface of the earth it is heaviest, and grows thinner and lighter in proportion to its distance from the earth.

As air has weight, like other fluids, it presses in every direction whatever bodies are immersed in it. This pressure is equal to a column of quicksilver, about 29 or 30 inches high. That is, a column of air, reaching to the extremity of the atmosphere, will support a column of quicksilver of equal diameter, at the height of

29 or 30 inches, as it will a column of water of equal diameter, at the height of 33 feet.

The quantity of the pressure of the atmospheric air is estimated to be not less than 2,232 pounds, upon every square foot of surface, or upwards of fifteen pounds upon every square inch. Computing, therefore, the surface of a man's body at 15 feet square, the whole pressure which it sustains is nearly equal to 33,480 pounds. By such a pressure, we should, undoubtedly, be crushed in a moment, if every part of the human body were not filled with air, or some other elastic fluid, the spring of which is strong enough to counteract that pressure.

But this pressure of the air answers many useful ends. It prevents the blood vessels in animals, and the sap vessels in plants, from being too much distended by the expansive power, which has a perpetual tendency to swell them out. It is owing to the gravity of air, that liquid substances remain liquid; for without that pressure, they would become vapour. Salts and oils remain united in air, but separate as soon as that fluid is extracted.

The absolute extent to which the atmosphere reaches from the earth, has not yet been determined. But the beginning and ending of twilight show, that the height at which the atmosphere begins to refract the beams of the sun, is about forty-four English miles; so that it is supposed to extend on all sides from the earth about fifty miles.

An instrument to show the different weight of the air, at different times, and at different dis

tances from the earth's surface, was invented by an Italian, named Torricelli. He first showed, in the year 1645, that a column of atmospherical air may be made to balance a column of another fluid having the same base. To avoid the inconvenience of using a pipe 30 or 33 feet, the length of the column of water which the pressure of the air was found to sustain, he employed quicksilver instead of water; because quicksilver being much heavier than water, the pressure of air would support a column of that metallic fluid, proportionably shorter. By this experiment it was found that the pressure of the air which elevates water to the height of 32 feet, will not sustain a column of quicksilver above the height of 28 or 30 inches. Upon this idea was constructed the instrument called barometer or measurer of weight. It is nothing more than a glass tube with a bulb at one end filled with' mercury. This is fastened to a piece of wood or metal graduated, in order to measure the rising or falling of the mercury, as it is pressed by the different weight of the air.

The thermometer, or heat measurer, is an instrument invented to show the different degrees of heat prevailing in air, water, or any other substance. Air, alcohol, and oil, have been successively employed as measures of heat; but quicksilver has been preferred to these, because it was found to be more regular in its expansions. A certain quantity of quicksilver is poured into a glass tube, which ends in a bulb or globe. It is so contrived that the part of the tube not filled with quicksilver, shall be empty of air, and