In 1658, Hooke finished an air-pump for Mr. Boyle, in whose laboratory he was an assistant: this pump had two barrels, which was afterwards improved by Hauksbee, and remained in common use till the introduction of Smeaton's pump.

About this period the Marquis of Worcester invented a steam-engine; at least he threw out the hint by which others have been able to construct the machine, though it should seem from his own account, in the "Century of Inventions," that he had actually worked an engine himself.

Little was done in the way of practice with the Marquis of Worcester's ideas, till the year 1700, when Savary constructed engines on his plan; and, in ten years afterwards, the piston and cylinder were invented by Newcomen: these, with the invention of Beighton's apparatus for turning the cocks by its own motion, made the machine such as it remained nearly stationary for many years.

The commencement of the modern experimental improvements in hydraulics, has been dated from the investigations of Mr. Smeaton, respecting the effects of wind and water, an account of which was published in the Philosophical Transactions for 1759; and in 1769, Mr. Watt obtained a patent for his improvements of the steam-engine, which includes almost every essential change that has been made since the time of Beighton.

A fluid has been defined to be a body whose parts are put in motion one among another by any force impressed upon it; and which, when the impressed force is removed, restores itself to its former state.

Another definition is, that a fluid is a collection of material particles infinitely small, and moving freely on each other, in every direction, and without friction.

Fluids are divided into elastic, and non-elastic: the former are those whose dimensions are diminished by increasing the pressure upon them, and increased by diminishing the said pressure; such are atmospheric air and the different gases.

The latter are those, whose dimensions are not, at least as to sense, affected by any increase of pressure; as water, oil, mercury, spirits, &c.

The science, which treats of the nature and properties of fluids, has been generally divided into three branches, viz. Hydrostatics, which comprizes the doctrine of the equilibrium of non-elastic fluids; Hydraulics, which relates to the motion of those fluids; and Pneumatics, which treats on the properties of the different kinds of air. It is not unusual to include all these under the general term of Hydrostatics.

A fluid, which has no immediate tendency to expand when at liberty, is usually considered as a liquid; such are water, oil, and mercury: the atmospheric air, chemical gases, and steam, are fluids, but not liquids.

The specific gravity of a body is its weight, when compared with the weight of another body, whose magnitude is the same. And the density of a body is as the quantity of matter contained in a given space, and is, therefore, in proportion to its weight, when the magnitude is the same: accordingly the specific gravity of a body is in proportion to its density.

Example. A cubical inch of pure mercury is about 14 times heavier than the same quantity of water, of course the density and specific gravity of the former are 14 times greater than those of the latter.

The cause of fluidity is not perfectly known: some persons say that the particles of fluids are spherical, and accordingly touching only in points, have very little cohesion, and easily slide over each other. In reply to this, it is observed, that the particles of fluids are probably of the same nature or figure as those of solids, because they are perpetually changed into one another; as water into ice, and solid metals into liquids by heat. Among many modern philosophers, it is assumed, that a certain portion of heat, combined in some way or other with bodies, occasions fluidity; and that the relative proportions of heat contained in fluids and solids, is the cause of the difference between them.

spaces Ex and Dz; so that, as action and re-action are equal, though in contrary directions, the pressure on the bottom will be the same as if the parts r and z were removed, and the whole vessel EBCD were filled with water. Or if holes were bored at r and z, the pressure of the water upwards would cause it to ascend till the fluid was at a level in the three compartments of the vessel, with which there then would be a communication.

This leads to what has been denominated the hydrostatical paradox, which is of great importance in the science we are now explaining, viz. “That a quantity of fluid, however small, may be made to counterpoise any quantity however large." This principle is explained in various ways by different authors, but it will be sufficiently evident by remarking, that if to a vessel of any size, a bended tube be cemented to communicate with it, and rise up on the outside of the vessel, and water be poured into either of them, it will rise to, and stand at the same height in both, consequently there will be an equi librium; and this will occur in all cases without regard to the shape of the vessels, nor does it at all signify whether the small tube stand parallel with the larger vessel, or whether it be inclined in any angle whatever. The general, and undeviating principle is, that water will find its level; hence this fluid, so important to mankind, may be carried, by means of pipes, to any distance, through vallies and over hills, provided those hills be not higher than the head whence it flows.

Another principle in hydrostatics is, that the pressure of a fluid is in proportion to its perpendicular height, and the base of the vessel containing it, without any regard to the quantity. Hence vessels have been burst, simply by fixing a small tube of considerable length in the top of them, and keeping them filled with water, because the pressure downwards, upwards, and sideways, is as great, as if the vessel itself was as high as the tube is long.

The pressure on each square inch of the side of a vessel, or on each square foot of the bank of a river or reservoir of

water, continually increases in descending to the bottom. The sum of the pressures on all the parts of the side is estimated by taking the mean depth, that is the point which would be the centre of gravity of the surface: thus, if we had a cubical vessel filled with water, or any other liquid, the centre of gravity of each side being the middle point, the pressure on each of the upright sides would be half as great as the pressure on the bottom, that is, since the pressure upon the bottom is equal to the weight of the water, and that on one side is equal to half the weight of the water in the vessel, of course the pressure upon the four sides and bottom is equal to three times the weight of the liquid.

If two fluids are of different specific gravities, that is, that equal bulks of them have different weights, their opposite pressures will counterbalance each other, when their heights above the common surface are inversely as their specific gravities, for the greater density of the one will precisely compensate for its deficiency in height. Thus a column of mercury standing at the height of thirty inches in a tube, will support the pressure of a column of water, in another branch of the tube, thirty-four feet high, because the weight of thirty inches of mercury is equal to that of four hundred and eight inches of

water.

We shall now describe the method of obtaining the specific gravities of different bodies. The instrument, fig. 2, is called the hydrostatical balance; it does not differ very much from the common balance. To the beam, there are the two scalepans adjusted, which may be taken off at pleasure. There is likewise another pan at x, of equal weight with that at z, furnished with shorter strings, and a small hook, so that any body may be hung to it, and then immersed in a vessel of water, CD.

The water used in this business should be quite pure, and of the same degree of temperature, because the density of water differs according to the degree of its temperature, hot water being somewhat lighter than cold. In very accurate experi

is also a section devoted to the subjects of winds, vapours, and the formation of springs,

The same subjects are treated of more at large by the Rev. T. Parkinson, in a quarto volume entitled "A System of Hydrostatics." This is now generally bound up with the System of Mechanics by the same author, to which we have before referred. Both works are illustrated with numerous engravings; and with a map of the winds, shewing their currents, and in what parts of the world the monsoons, the variable and the shifting winds are to be met with, and in what months they prevail. Another valuable plate gives the graduations of fifteen different thermometers, by which the calculations of different authors to the boiling point may be easily reduced to one another, The last chapter in the volume treats in a concise but luminous manner of the motion of bodies in fluids.

PNEUMATICS.

THE science of pneumatics treats of the mechanical properties of the air, or æriform fluids; such as their weight, density, compressibility and elasticity.

The air is a ponderous fluid, in which we live and breathe; and which entirely envelopes the whole globe, extending to the height of from 40 to 50 miles above it on all sides. That it is a fluid body is evident, because all its parts are easily moved, and yield to the smallest inequality of pressure :-that it has gravity is proved from the following considerations, 1. It always accompanies the earth in its path round the sun, which indicates that it is connected with the earth by the general force of gravity. On this account it is continually moving round the earth; forming, in some parts where it is quite free, the trade-winds. 2. It is owing to its gravity that it supports clouds and vapours, which are constantly floating in it. We see occasionally balloons, to which substances of many hundred weight are attached, float, and even rise some miles in height in the air; which proves the gravity of air, in the same manner,