will flow off, till the vessel be completely emptied.

CAPILLARY ATTRACTION.

When water is poured into a vessel, or into any number of communicating vessels, its surface will be horizontal; or, it will rise to the same height in each vessel, whatever be its form or position. This is the fact, however, only when the diameter of the communicating vessels or tubes exceeds the fifteenth of an inch; for if a set of communicating vessels be composed of tubes of various diameters, the fluid will rise to a level surface in all the tubes which exceed one fifteenth of an inch in diameter; but if there be tubes of a smaller diameter in them, the fluid will rise above that level, to heights inversely proportional to the diameters of the tubes. The power by which the fluid is thus raised above its natural level, is called capillary attraction; and the glass tubes which are employed to exhibit its phenomena, are named capillary tubes. These appellations are derived from a Latin word, signifying a hair. This power has not yet been satisfactorily accounted for, though its operations have been traced and calculated.

The fluidity of water is increased by heat. It has been proved by experiment, that a jet of warm water will spring much higher than a jet of cold water, and that a syphon which discharges cold water only by drops, will discharge water of a high temperature in a continued stream.

This fact was observed by the ancients; for Plutarch remarks that the clepsydræ, or water clocks go faster in summer than in winter, and he attributes this to the influence of heat.

A clepsydra, or water clock, is a machine which measures the lapse of time by the descent of water in a vessel. If such a form be given to a vessel that the areas shall increase uniformly, as the times; then the times in which the surface of any fluid, contained in that vessel, descends, by running out through an orifice in its side or bottom, will be in the same ratio, and the vessel will form a machine for measuring time. If the vessel be cylindrical, and empty itself in twelve hours, its altitude may be divided in such a manner, that the fluid surface may take.exactly an hour to descend through each division.

In order to determine, with expedition, the strength of spirituous liquors, which is inversely proportional to their specific gravities, an instrument is employed, called, Hydrometer, or arcometer, or gravimeter; the invention of which is attributed, by some, to Archimedes ; by others, the honour is assigned to a female named Hypathia, who flourished towards the end of the fourth century.

Fahrenheit's hydrometer made of glass or metal, consists of a cylindrical stem graduated, and having two hollow balls of unequal sizes attached towards the same end of it. Into the lowermost of these, or that quite at the end, is introduced quicksilver enough to make itself and the other ball sink a little below the surface of distilled water. If this apparatus be plunged

into a fluid lighter than water, the larger ball will sink below the surface, and if it be immersed in a heavier fluid, it will rise nearer the surface.

QUESTIONS.

What are the specific gravities of bodies? In what manner is any body immersed in a fluid, or floating on its surface, pressed? What quantity of water is displaced by any body which swims or sinks in it? When a solid is weighed in a fluid, how much of its weight does it lose? When will a solid immersed in a fluid, sink, or float, or remain wherever it is placed? What is the proportion between two or more fluids, as to their specific gravities? What is the proportion between the specific gravity of a solid and that of a fluid? What is the proportion between the specific gravity of a solid floating in a fluid, and the specific gravity of the fluid itself? When are bodies said to be of the same bulk? What is the method of detecting the adulteration of the precious metals? How did Archimedes discover the adulteration of Hiero's golden crown? What is the rule for finding the specific gravities of such solids as sink in water? What fluid is generally used for weighing substances, in order to find their specific gravities? What is the hydrostatic balance, and by what other names is it called? What is a syphon? What is Fahrenheit's hydrometer? What is capillary attraction?

HYDRAULICS. Hydraulics is that branch of hydrodynamics which teaches how to estimate the velocity and force of fluids in motion. This,

consequently, comprehends the theory of running water, whether issuing from openings in reservoirs, impelled by the pressure of some substance lying above them, or rising perpendicularly from atmospherical pressure; or raised in pumps, or other machines, invented by

human art.

If water issue from an orifice in the side or bottom of a reservoir of any kind, its surface remains in a horizontal position, maintaining its level, till it nearly reach the bottom; all its particles descending in vertical lines. But then, the particles change the direction of their motion, and rush towards the opening, in different degrees of obliquity. If some small substances specifically heavier than water, be thrown into the fluid while it is running out through the orifice, they will, at first, descend vertically; but when they approach near to the bottom, they will deviate from this direction, and will describe oblique curves. A contraction in the vein or column of descending fluid is, also, manifest from observation. It was first observed by Sir Isaac Newton, and denominated the vena contracta, or the contracted vein.

The velocity with which water runs from an opening in the bottom or side of a vessel, is estimated to be proportionate to the square root of the distance of the surface of the water, to the opening.

For instance; if the velocity of water, running through an orifice, at the distance of one foot beneath the surface, be estimated at 1; then, if

flowing through an opening four feet beneath the surface, the velocity will be as 2; and if the hole through which it flows, be nine feet below the surface, the velocity will be as 3.

The lateral pressure of water, that is, its pressure against the sides of the reservoir containing it, is as the square of the depth of the reservoir.

Thus, in a vessel of water three feet deep, at the depth of one foot, the pressure of the fluid against the sides will be 1; at the depth of two feet, it will be 4, and at the depth of three feet, it will be nine.

By means of its property of pressing equally every way, water may be conveyed in pipes, over hills and through valleys, to any distance, and to any height, not exceeding that of the spring whence it first issues.

This is the principle upon which water-works and aqueducts are constructed.

When a fluid issues vertically, from any containing reservoir, it will rise to a height equal to the perpendicular distance of the orifice by which it issues, from the surface of the fluid. Owing, however, to the resistance of the air, and the friction of the issuing fluid against the sides of the orifice, jets of water do not rise to exactly that height.

The oscillations of water in a syphon, consisting of two vertical branches and a horizontal branch, are isochronous, that is in equal times; and they have the same duration as the oscillations of a pendulum, whose length is equal to