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leagues from the land. Their change is at the vernal and autumnal equinoxes, and it is accompanied with terrible ftorms of thunder, lightning, and rain. The monsoons are occafioned by the cold air moving towards thofe places, in which the air is rarefied by the heat of the fun, in order to restore its equilibrium. 3. The land and fea breezes, which are periodical winds, and blow from the land from night to about mid-day, and from the sea from about noon to mid-night. Thefe winds do not extend above two or three leagues from the shore.

Beyond the latitude of thirty degrees, north and fouth, the winds, as we daily perceive in Great Britain, are more variable, though it may be obferved in general, that the tendency of the wind is from a colder region to that which is hotter.

CHA P. CVIII.

OF MISTS, CLOUDS, RAIN, DEW, SNOW, AND

MISTS

HAIL.

STS are those collections of vapours, which chiefly rise from fenny moist places, and become more visible as the light of the day decreases.

Clouds

Clouds are nothing else but a collection of moist particles, exhaled from the fea and earth by the heat of the fun, fufpended aloft in the air, and foaring on the wings of the wind.

The height of the clouds is fupposed to be from about a quarter of a mile, to a mile. It is common for perfons, by climbing very high mountains, to get above the clouds, and fee them fwim beneath: them.

The wonderful variety in the colour of the clouds, is owing to their particular fituation with regard to the fun, and the different reflections of his light. The different figures of the clouds refult from their loofe and voluble texture, revolving into any form according to the different force of the winds.

Rain is nothing but thick clouds condenfed by the cold, which by their own weight fall upon the earth in fmall quantities, called drops of water.

Thofe fmall clouds, fometimes feen very high, and heaped upon one another, prefage rain very foon..

When the horizon, at the rifing or fetting of the fun, appears pale and yellowish, it is a fign of the air being full of vapours, and threatens bad weather. But when it is of a light red at those times, there are but few vapours in the air, and fine weather may be expected..

If the cloud that melts is greatly rarefied, and its particles, in falling, meet an air moderately warm, these drops will be so small, that they will not compofe rain, but rime only.

Dew is produced from a quantity of particles of water extremely fubtile, that float about in a calm and ferene air in form of vapours, which, being condensed by the coldness of the night, lofe by degrees their agitation, and many uniting together, fall in the morning in small invisible particles, like an extremley fine and delicate rain, which continues but a fhort time, and is feen in drops of water like pearls upon leaves and herbs.

Snow is produced thus. In winter the regions of the air are intenfely cold, and the clouds finding this great cold on every fide, quickly pass from that state of condensation, which might reduce them to rain, into that which is able to reduce them to ice; fo that in winter, as foon as the clouds begin to change into very fine drops of water, each of thefe fmall particles freeze, and touching each other, form flakes of fnow.

The fmall intervals that the flakes leave between them, like fo many pores, filled with a fubtile air, are the cause of their lightness.

The fnow is white, because the small particles of ice, which compose those flakes, being hard, folid,

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transparent, and differently arranged, reflect to us the light from all parts.

Hail is formed, when the parts of a cloud, beginning to fall, meet in their defcent a very cold air, which freezes them; and these small bits of ice are very near the figure and fize that the drops of water would have been, had they fallen.

CHAP. CIX.

OF THE SEVEN COLOURS, AND THE RAINBOW.

HERE are no more than feven primary, or

TH

original colours, namely, red, orange, yellow, green, blue, indigo, and violet. All the other colours are compofed of some of these primary ones.

The difference of colours is owing to the different refrangibility of the rays of light, which produce them. By the refrangibility of the rays of light, I mean their tendency to be turned out of a straight line, or to be rendered crooked, in paffing through any medium. The leaft refrangible rays are those that produce red; and the most refrangible, are thofe that produce violet. All the intermediate rays produce

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produce the intermediate colours in the order abovementioned.

If the rays of the fun were not of different colours, there would be only one colour in the world; for the colours are in the rays of light, and not in the bodies that are dyed with them. That this opinion is well founded, we may be convinced by observing, that all bodies are of the fame hue in the dark. Perhaps it may be said, if it be absolutely dark, we cannot fee at all. True. But there fure may be something between an obfcure light and abfolute darkness, just sufficient to fhew us, that a man has got a great coat upon his back, though we cannot tell for certain, of what colour it is.

Black and white are not original colours. White is a mixture of all the primary colours; and black is a privation of them all, or no colour at all.

The iris, or rainbow, is a beautiful arch in the heavens, ornamented with all the primary colours. It is only feen when the fpectator turns his back to the fun, and when it rains on the opposite fide.

Its beautiful colours ftruck antiquity with amazement. To the philofophers Pliny and Plutarch, it appeared as an object which we might admire, but could never explain.-The priests always preferred the wood, on which the rainbow had appeared to reft, for their facrifices, vainly fuppofing that this wood

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