among ecclesiastical writers, the expelling devils from persons possessed, by means of conjurations and prayers.

EXPERIMENTAL PHILOSOPHY;

That philosophy which proceeds on experiments, which deduces the laws of nature, and the properties and powers of bodies, and their actions upon each other, from sensible experiments and observations.

I. It is not very long since this science has been known to the world, or, to speak more properly, since it was first reduced into a system. Natural philosophy has been, for these 50 centuries, nothing more than a confused heap of systems laid one upon another, and very frequently the one clashing against the other. Each philosopher thought, that he had an equal right to erect a similar edifice to his own memory. They adopted barbarous terms and expressions, that conveyed confused ideas only. For explications, they gave certain unintelligible or unmeaning words, which had been introduced by the authority of some celebrated name, but from which a man of understanding could not receive the least information. At length, the true physics was brought to light; it was drawn from the obscurity of the schools, where it had grown old under the authority of Aristotle, and scarce anything been suffered to remain of it but the name. This reformation proceeded principally from the manner of studying it. Instead of guessing at it, they began to investigate it by experiments; and whereas they formerly confined themselves to speculations, and vague researches concerning phenomena and their causes, that were always merely conjectural, they now gave ocular demonstrations of causes and effects by means of experiments; and this is what they call experimental philosophy.

II. The principles of this philosophy are as follow. All the material substances, whose assemblage composes the universe, are called natural bodies. What we perceive in these substances that is uniform and inviolable, and of which we do not know the cause, is called their properties. Physics sets out with this, as from a fixed point, in order to explain the different phenomena that are perceived on the earth, in the water, the air, or fire, and in all that these elements contain. For though it does not pretend to know all that bodies have in common among themselves, or all that is peculiar to each one of them; yet it knows a certain number of their properties, which it regards as primary, till it discovers a precedent cause of which they may be the effects; and which properties are general, and in a manner inseparable from all matter, as for example, extension. There are likewise properties of an inferior order, which do not appertain to all bodies but as they are in certain states, or under certain circumstances; these, in general, are nothing more than combinations of the primary properties; and form a second class, as for example, fluidity. Lastly, these properties of the first and second order combine more and more, and become common to a still smaller number of bodies; and here they are no longer extended to all bodies, as the first; nor are peculiar to certain states, as the second; but are confined to genders, species, or even individuals. Such are several properties of the air, fire, light, metals, the magnet, &c. These three orders of properties are the subject of the inquiries of experimental philosophy, which proves by experiments those that are already known, and frequently discovers others that were unknown.

III. It is necessary here to descend to some particulars. The first property of bodies, which presents itself to our ideas and our senses, is their extension; which is a limited bulk of any form whatever, of which we can conceive parts that may be distinguished from each other. This material extension has three dimensions, which are length, breadth, and depth. Every body, whose extension is large enough to be seen or felt, may be divided into several parts, and which must consequently decrease in proportion as the division is increas- by which certain bodies, when compressed, endeavour to restore themselves to their former state; and this property supposes them compressible. As all these natural properties of bodies are of great utility in explaining the principles of physics, and in applying them to all the arts, experimental philosophy proves their reality by a thousand examples.

V. We discover still other properties in bodies; such as mobility, which we must not here confound with motion. This mobility arises from certain dispositions which are not in an equal degree in all bodies; from whence it comes that some are more easily moved than others; and this proceeds from the resistance to motion which is perceived in all bodies, having regard merely to their masses; and this resistance is called vis inertiae, or inert force. A body is said to be in motion, when it is actually moving from one place to another; or, whenever a body changes its situation with regard to the objects that surround it, either nearly or remotely, it is said to be in motion. There are three principal matters to be considered in a moving body; its direction, its velocity, and the quantity of its motion: and here physics explains the force or moving power; it likewise distinguishes between simple and compound motion. Simple motion is that which arises from only one force, or which tends to only one point. It describes the laws, and explains the resistance, of mediums; the resistance of friction; the difficulties of a perpetual motion; the alteration of direction, occasioned by the opposition of a fluid matter; reflected or reverberated motion; the communication of motion by the shock of bodies, &c. Compound motion is that of a body impelled to move by several causes or powers which act according to their different directions. Physics here likewise investigates the laws of motion; and is particularly applied to the explaining, under this head, what are called the central forces, which produce a motion that is either circular or in a curve line, and which incessantly urge the moving body either to approach or recede from the centre. To distinguish these from each other, the former is called the centrifugal force, and the latter the centripetal force.

VI. By gravity, or ponderosity, is to be understood that force which occasions bodies to pass from a higher to a lower place, when nothing opposes their course, or when the obstructions are not sufficient to stop them. Speculative philosophy investigates its cause, and perhaps in vain. Experimental philosophy contents itself with describing the phenomena, and teaching the laws of gravity, which are thoroughly established by a thousand reiterated experiments. In order properly to understand this subject, we must take care not to confound the term gravity with that of weight. By the former, we understand that force which urges bodies to descend through a certain space in a given time. By the latter, is meant the quantity of a heavy body that is contained under the same bulk. The phenomena are explained by the experiments themselves, and by inferences deduced from them.

VII. Hydrostatics is a science whose object is the gravity and equilibrium of fluids in particular. Tho' the gravity of these bodies is the same with that of others, and is subject to the same laws, yet their state of fluidity gives rise to particular phenomena, which it is of consequence to know. But as hydrostatics cannot be successfully treated on without the assistance of calculation, it has been ranged among the mathematical sciences. See Mathematics.

VIII. We say the same with regard to mechanics; which is the art of employing, by the aid of machines, the motion of bodies, in conformity to its properties and laws, as well with regard to solids as fluids, either more commodiously or more advantageously.

IX. After it has made the most accurate experiments, and the most judicious observations, on all these different subjects, and the properties of bodies in particular, experimental philosophy passes to the examination of the air, the water, fire, the wind, colours, &c. The air is a fluid with which we are surrounded from the instant of our birth, and without which we cannot exist. It is by the properties and the influences of the air, that nature gives increase and perfection to all that it produces for our wants and conveniences; it is the spirit of navigation: sound, voice, speech itself, are nothing more than vibrations of the air: this globe that we inhabit is completely surrounded by air; and this kind of covering, which is commonly called the atmosphere, has such remarkable functions, that it evidently appears to concur to the mechanism of nature. Experimental physics, therefore, considers the air, 1. Of itself, independent of its bulk, and the figure of its whole body: it examines its essential properties; as its gravity, density, spring, &c. The air-pump is here of indispensible use; and by this machine physics examines in what manner space, or a vacuum, is made. It likewise shows the necessity of air to the preservation of animal-life; the effect it has on sound, fire, and gunpowder, in vacuo; and a hundred other experiments of various degrees of curiosity. 2. It considers the air as the terrestrial atmosphere, sometimes as a fluid at rest, and sometimes as in motion. And by these means it accounts for the variation of the mercury in the barometer, and why it sinks in proportion as the height of the atmosphere diminishes; as also for the figure, the extent, and weight of the atmosphere: it shows the method of determining the height of mountains, the nature of sound in general, of its propagation, and of sonorous bodies.

IX. It is here also, that experimental philosophy considers the nature of the wind; which is nothing more than agitated air, a portion of the atmosphere that moves like a current, with a certain velocity and determinate direction. This fluid, with regard to its direction, takes different names according to the different points of the horizon from whence it comes, as east, west, north, and south. Winds are likewise distinguished into three sorts; one of which is called general or constant, as the trade-winds, which continually blow between the tropics: another is the periodical, which always begin and end within a certain time of the year, or a certain hour of the day, as the monsoons, the land-breezes and sea-breezes, which rise constantly in the morning and evening; and lastly, such as are variable, as well with regard to their direction as their velocity and duration.

M. Mariotte computes the velocity of the most impetuous wind, to be at the rate of 32 feet in a second, and Mr Derham makes it 66 feet in the same time. The first, doubtless, meant the wind of the greatest velocity that had then come to his knowledge. X. The force of the wind, like that of other bodies, depends on its velocity and mass; that is, the quantity of air which is in motion: so the same wind has more or less force on any obstacle that opposes it, in proportion as that obstacle presents a greater or a less surface: for which reason it is that they spread the sails of a vessel more or less, and place the wings of a windmill in different directions. The machines, by which the winds are measured, are called anemometers. They show the direction, the velocity, and the duration of winds. It is by the agitations of the wind, that the air is purified; that the seeds of trees and herbs are conveyed to the forests and fields; that ships are driven from one pole to the other; that our mills turn upon their axes, &c.; and art, by imitating nature, sometimes procures us artificial winds, by which we refresh our bodies, invigorate our fires, purify our corn, &c.

XI. Water is an universal agent, which nature employs in all her productions. It may be considered as in three states, 1. As a liquid; 2. As a vapour; 3. As ice. These three different states do not in any manner change its essence, but make it proper to answer different ends. The natural state of water would be that of a solid body, as fat, wax, and all those other bodies which are only fluid when heated to a certain degree: for water would be constantly ice, if the particles of fire, by which it is penetrated in the temperate climates, did not render it fluid, by producing a reciprocal motion among its parts; and, in a country where the cold is continually strong enough to maintain the congelation, the assistance of art is necessary to make it fluid in the same manner as we do lead, &c. Water, when not in ice, is a fluid that is insipid, transparent, without colour, and without smell, and that easily adheres to the surface of some bodies, that penetrates many, and extinguishes fire. Experimental philosophy investigates the origin of fountains; the cause of the saltness of the sea; the means of purifying water; what is its weight, and what are its effects when heated, &c. It likewise examines this fluid in the state of vapour; and, finds that a drop of water, when in vapour, occupies a space vastly greater than it did before. It explains the volatile and its effects; fire engines; and the force of vapours that give motion to immense machines in mines and elsewhere, &c. and lastly, it considers water in the state of ice. Ice consequently is more cold than water; and its coldness increases if it continue to lose that matter, already too rare, or too little active, to render it fluid. Experimental physics endeavours to investigate the causes of the congelation of water, and why ice is lighter than water; from whence it derives that expansive force by which it breaks the containing vessel; the difference there is between the congelation of rivers and that of standing waters; why ice becomes more cold by the mixture of salts; and many other similar phenomena.

XII. The nature of fire is yet very much unknown to the most learned philosophers. As objects when at a great distance are not perceptible to our senses, so when we examine them too nearly, we discern them but confusedly. It is still disputed whether fire be a homogeneous, unalterable matter, designed, by its presence, or by its action, to produce heat, inflammation, and dissolution, in bodies; or if its essence consists in motion only, or in the fermentation of those particles which we call inflammable, and which enter as principles, in greater or less quantities, in the composition of mixed bodies. The most learned inquirers into nature incline to the former opinion; and to have recourse to a matter, which they regard as the principle of fire. They suppose that there is in nature a fluid adapted to this purpose, created such from the beginning, and that nothing more is necessary than to put it in action. The numberless experiments which are daily made in electricity, seem to favour this opinion, and to prove that this matter, this fluid, this elementary fire, is diffused through all nature, and in all bodies, even ice itself. We cannot say to what important knowledge this great discovery of electricity may lead if we continue our inquiries concerning it. It appears, however, that we may believe, without any inconvenience or absurdity, that fire and light, considered in their first principle, are one and the same substance differently modified.

XIII. Be this matter however as it may, experimental philosophy is employed in making the most ingenious and most useful researches concerning the nature of fire, its propagation, and the means by which its power may be excited or augmented; concerning the phosphorous and its inflammation; fire excited by the reflection of the sun's rays from a mirror; and on the effects of fire in general; concerning lightning and its effects; the fusion of metals; gunpowder and its explosion; flame, and the aliment of fire; and an infinity of like objects which it explains, or concerning which it makes new discoveries, by the aid of experiments.

XIV. By the word light, we understand that agent by which nature affects the eye with that lively, and almost constantly pleasing sensation, which we call seeing, and by which we discern the size, figure, colour, and situation of objects, when at a convenient distance. All philosophers agree, that the light, which is diffused in any place, is a real body. But what this body is, and by what means it enters that place where it is perceived, is a question about which philosophers are divided.

XV. Experimental philosophy is applied in discovering or proving, by an infinity of experiments, what is the nature of light, in what manner it is propagated, what its velocity and progressive motion. It also investigates and explains the principles of optics, properly so called, and shows the directions which light observes in its motions. From thence it proceeds to the examen of the principles of catoptries, and describes the laws and effects of reflected light. It next treats of the principles of dioptries, and explains the laws of refracted light; and lastly, it teaches, from the principles of natural and artificial vision, the construction of optical instruments, as lenses, concave mirrors, prisms, telescopes, &c. &c. and the uses to which they are applied.

XVI. By resolving or separating the rays of light, philosophy has obtained true and clear discoveries of the nature of colours. We are naturally led to imagine that colours, and their different degrees, make a part of the bodies that present them to our sight; that white is inherent in snow, green in leaves and grass, and red in a stuff dyed of that colour. But this is far from being true. If an object, which presents any colour to our sight, be not illuminated, it presents no colour whatever. In the night all is black. Colours, therefore, depend depend on light; for without that we could form no idea of them: but they depend also on bodies; for of several objects presented to the same light, some appear white, others red, blue, &c. But all these matters being separate from our own bodies, we should never acquire any ideas of them, if the light, transmitted or reflected by these objects, did not make them sensible to us, by striking upon the organs of our sight, and if these impressions did not revive in us those ideas which we have been used to express by certain terms. For these reasons philosophy considers colours from three points of view, 1. As in the light; 2. In bodies, as being coloured; and, 3. From the relation they have to our visual faculties, which they particularly affect, and by which we are enabled to distinguish them.

It is unnecessary in this place to say more either on colour in particular, or experimental philosophy in general. The different subjects of this collective article are particularly treated under their proper names, in the order of the alphabet: the reader will therefore turn, as he has occasion, to Acoustics, Catoptrics, Chromatics, Dioptrics, Hydrostatics, Mechanics, Optics, Pneumatics, Electricity, Magnetism, &c. &c. &c.—Also Air, Atmosphere, Burning-Glass, Cold, Colour, Congelation, Evaporation, Fire, Flame, Fluidity, Heat, Ignition, Light, Sound, Steam, Water, Wind, &c.

**EXP**