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In the temperate regions of the earth, where the heats are irregular, currents of air or winds will of course be variable, and proceed from all directions at different times; but some local circumstances may occasion certain winds to be more prevalent in one country than in another ; thus, in Britain, the south-west winds are observed to be much more fre. quent than those from any other quarter.
In certain parts of the sea, at and on each side of the equator, where the heat of the sun occasions the most sensible and the most uniform changes in the temperature of the air, the rarefaction following the sun's course, the colder air rushes in from the eastward, and hence produces currents in general from east to west, during the whole year; to which, from their great utility to vessels employed in commerce, we have given the name of the trade-winds. It is to gain the assistance of this wind that vessels from Europe, bound for the West India islands, usually run a good way southwards on the coast of Africa, although by this they lengthen their voyage ; for when they have come within the range of the trade-winds, they are sure of finding a fair wind to carry thein to their desired port. In the Indian ocean the tradewinds blow regularly from east to west between 30° and 10° of south latitude: but to the northward of this line, on both sides of the equator, the wind changes every six months, one half of the year blowing, in a general sense, from east to west, and the other half of the year from west to east. These changing trade-winds are called monsoons.
In warm climates we meet with what are called sea and land breezes ; that is, the wind, during a certain portion of the day, blows on shore from the sea, and in the night time off shore from the land. The cause of this phenomenon is supposed to be, that the land being more heated by the sun's rays than the sea, the air upon the land is heated and rarefied; and, consequently, the cooler air from the sea crowds in to fill its place : but in the night time, when the
air on the land is cooled, the equilibrium is restored, and the current of air again returns to the sea.
Besides these motions of the air, by which great masses are observed to change their relative positions, there is another motion of the partieles of air among themselves, or a sort of vibration by which they move forwards and backwards for some time, in proportion to the strength of the impulsę at first applied. This vibration is occasioned by a stroke on such bodies as we call sonorous, whose parts being elastic, move forwards through a small space, and then return to their original position ; but without stopping there, they move backwards nearly as far as they moved forwards, and thus continue this vibratory motion, gradually diminishing in effect until at last they come to be perfectly at rest : such a motion is observed in a weight fixed at the end of a string, and suspended from the finger. This vibration of the parts of a sonorous body must act upon the particles of air in contact with them, by successive, but gradually diminishing impulses, and thus by agitating the membrane called the tympanum, or drum of the ear, occasion in us that sensation to which we give the name of sound. Although these vibrations are produced in succession, yet the intervals between the vibrations are so minute as not to be perceptible, and therefore to our senses they produce only one constant uniform sound.
When these vibrations of the air strike against any fixed immoveable surface or body, they are compelled to change their course and move backwards, thus acting again on the ear, and producing a second sound, which we call an echo,
The nature and process of vibration of the particles of air, may easily be conceived by observing the appearances and effects produced by throwing a stone into a pond of still water. The water suddenly displaced by the stone rises up in a circular wave round it, and then sinks down into a
hollow nearly as much below the surface of the pond as the top of the wave was above it : again, another wave is produced and succeeded by another hollow; and in this manner a number of concentric waves and hollows are exhibited, gradually diminishing in elevation and depression, until the force of the impulse communicated by the stone is exhausted, and the circles totally disappear; but, if the impulse has been considerable, these circles are continued 10 the margin of the pond, where, striking against the banks, a contrary impulse is given to the water, and a fresh series of waves and depressions is produced, returning back towards the centre of the pond, in the same manner as the echo returns to the ear, by the repercussion of the particles of air from the resisting body, by which the vibrations of the air had been interrupted.
Sounds are observed to move with an equable velocity of about 1142 feet in one second of time : but the strength or loudness of sounds vary agreeably to the state of the atmospbere. In cold dense air sounds are loudest, and in warm, thin, or rarefied air, they are weakest : when all air has been extracted from a vessel, or a vacuum is produced, no sound is perceived.
It was already observed, that the atmosphere is necessary for the transmission of light, which is composed of inconceivably minute particles, proceeding in straight lines, and in all directions, from certain bodies, which are thence said to be luminous, such as the sun and other celestial bodies, a fire, a candle, or any other substance in a state of combustion or burning, from heat in a certain temperature, as red hot iron ; when a flint is struck against another flint, or against a steel, luminous sparks are produced.
Light is so generally found united with heat, that we are apt to consider then as inseparable ; but light is frequently perceived when there is no sensible heat, as in the appearance of puttescent animal and vegetable substances, of glow
worms and other insects, of the sea when moved by the dashing of the oar, or when it breaks in waves along the beach. Light without heat is also, in certain circumstances, emitted from the diamond.
The motion of light is prodigiously rapid, being calcu. lated at ucarly 200,000 miles in one second of time,
When a ray of light passes through a substance of one uniform density, as through a pane of smooth glass, or when it passes perpendicularly from one medium to another, it moves forward in the same direction as before it touched the medium ; but when a ray passes obliquely from one medium to another of a different density, it is turned aside from its former direction, and is then said to be refracted ; and when a ray of light falls on an opposing body which it cannot penetrate, it is thrown back, and is hence said to be reflected; the reflected ray always forming, with the surface of the reflecting body, an angle equal to that formed with the same body, by the original impinging ray; or, in other words, the angle of reflection is always equal to the angle of incidence. Thus, if the original ray fall perpendicularly on the reflecting budy, the reflected ray will return in a perpendicular direction to the object from which the original ray proceeded : but, if the original ray fall on the reflecting body at any angle, as 45 degrees, the reflected ray will fly off at an equal angle of 45 degrees from the opposite part of the reflecting body. The truth of these observations may easily be ascertained by a person observing his face in a mirror; for if he look perpendicularly at the glass he will sce his face reflected, but if he stand on one side, and look obliquely at the glass, he no longer observes bis figure, which, however, will be perceived by another person looking at the glass with an angle of equal obliquity, in an opposite direction. When a boy drops his marble on a level stone, it rises perpendicularly towards his hand: but if he throw it with any inclination against the stone, the marble flies off
in an opposite direction, making the angle of reflection equal to the angle of incidence.
When rays of light proceed in all directions from a luminous body, they are said to diverge ; and when, as by a concave mirror, they are collected together at one point, they are said to converge, and that point is called the focus, where are assembled the whole particles of light and heat reflected from the concave surface of the mirror, which, from this circumstance, is commonly called a burning glass,
From observing the effects produced on the rays of light, by their transmission through a medium whose opposite surfaces are convex, or portions of spheres swelling outwards, common spectacles and telescopes have been constructed; some of these last possessing the power of enlarging the images of distant objects to a very high degree.
Although light appears in general white, yet it is composed of rays of various colours, as red, orange, yellow, green, blue, purple, and violet, as may be observed by transmitting a ray of the sun's light into a dark room, through a prism of pure glass; for the different parts of rays being susceptible of different degrees of refraction, these parts will be separated in passing through the prism, and exhibit on a sheet of paper the colours peculiar to each part of the white ray. It is by the refraction of the sun's rays in passing through the minute globules of which vapours and clouds consists that the beautiful arch in the heavens, to which we give the naine of the rainbow, is produced.
It was before noticed, that light and heat are by no means inseparable: heat is a substance by whose action fluids are converted into vapour, and solids are either converted into vapour, rendered fluid, changed into glass, or undergo uther processes observable in a state of combustion. By light we acquire the sensation of vision ; by heat that of warmth, or of heat itself. By heat bodies are expanded or enlarged; VOL.II.