there be a pressure of one pound, another wind, with double the velocity, will press the board with a force of four pounds, &c. The following table, extracted from the Philosophical Transactions, shows the velocity and pressure of the winds according to their different appellations.

CHAPTER X.

Of Vapours, Fogs and Mists, Clouds, Dew and HoarFrost, Rain, Snow and Hail, Thunder and Lightning, Falling Stars, Ignis Fatuus, Aurora Borealis, and the Rainbow.

1. VAPOURS are composed of aqueous or watery particles, separated from the surface of the water, or moist earth, by the action of the sun's heat; whereby they are so rarefied, attenuated, and separated from each other, as to become specifically lighter than the air; and, consequently, they rise and float in the atmosphere.

2. FOGS AND MISTS. Fogs are a collection of vapours which chiefly rise from fenny, moist places, and become more visible as the light of the day decreases. If these vapours be not dispersed, but unite with those that rise from water, as from rivers, lakes, &c. so as to fill the air in general, they are called mists.

3. CLOUDS are generally supposed to consist of vapours exhaled from the sea and land. These vapours ascend till they are of the same specific gravity as the surrounding air: here they coalesce, and by their union become more dense and weighty. The more thin and rare the clouds are, the higher they soar, but the height

* Dr. Thompson, in vol. iv. of his Chemistry, page 79, &c. edition of 1810, says, it is remarkable that, though the greatest quantity of vapours exists in the lower strata of the atmosphere, clouds never begin to form there, but always at some considerable height. The heat of the clouds is sometimes greater than that of the surrounding air. The formation of clouds and rain is neither owing to the saturation of the atmosphere, nor the diminution of heat, nor the mixture of airs of different temperatures. Evaporation often goes on for a month together in hot weather, especially in the torrid zone, without any rain. The water can neither remain in the atmosphere, nor pass through it in a state of vapour: What then becomes of the vapour after it enters the atmosphere? What makes it lay aside the new form which it must have assumed, and return again to its state of vapour, and fall down in rain ? Till these questions are experimentally answered, Dr. Thompson concludes, that the formation of clouds and rain cannot be accurately ac counted for.

seldom, if ever, exceeds two miles. The generality of clouds are suspended at the height of about a mile; sometimes, when the clouds are highly electrified, their height is not above seven or eight hundred yards. The wonderful variety in the colour of the clouds is owing to their particular situation to the sun, and the different reflections of his light. The various figure of the clouds probably proceeds from their loose and voluble texture, revolving in any form according to the different force of the winds, or from the electricity contained in them.

The general colour of the sky is blue, and this is oc casioned by the vapours which are always mixed with air, and which have the property of reflecting the blue rays more copiously than any other.*

4. DEw. When the earth has been heated in the daytime by the sun, it will retain that heat for some time after the sun has set. The air being a less dense, or less compact substance, will retain the heat for a less time: so that in the evening, the surface of the earth will be warmer than the air about it, and consequently, the vapours will continue to rise from the earth; but, as these vapours come immediately into a cool air, they will only rise to a small height; as the rarefied air in which they began to rise becomes condensed, the small particles of vapours will be brought nearer together. When many of these particles are united, they form dew; and, if this dew freeze, it will produce hoar-frost.

5. RAIN. When the weight of the air is diminished, its density will likewise be diminished, and consequently, the vapours that float in it will be less resisted, and begin to fall, and, as they begin to strike one upon another in falling, they will unite and form small drops. But when the small drops of which a cloud consisted are united into such large drops, that no part of the atmosphere is sufficiently dense to produce a resistance able to support them, they will then fall to the earth, and constitute what we call rain. If these drops be formed in the higher regions of the atmosphere, many of them will be united before they come to the ground, and the drops

* Saussure, Voyages dans les Alpes, vol. iv. p. 288.

of rain will be very large. The drops of rain increase so much both in bulk and motion, during their descent, that a bowl placed on the ground would receive in a shower of rain, almost twice the quantity of water that a similar bowl would receive on a neighbouring high steeple. The mean annual quantity of rain is greatest at the equator, and decreases gradually as we approach the poles. Thus, at

Grenada, West-Indies,

Latitude.

St Domingo, Cape St. François, 19° 46′

Cal cutta,

In England,
Petersburg,

12o 0'

Depth of rain. 126 inches. 120

22° 23′- 81

On the contrary, the number of rainy days is smallest at the equator, and increases in proportion to the distance from it. The number of rainy days is often greater in winter than in summer; but the quantity of rain is greater in summer than in winter. More rain falls in mountainous countries than in plains. Among the Andes, it is said to rain almost perpetually, while in the plains of Peru and in Egypt, it hardly ever rains at all. The mean annual quantity of rain for the whole globe is estimated by Dr. Thompson at 34 inches in depth, hence, may be found the whole quantity of rain that falls in a year upon the whole surface of the earth and sea, in the same manner as the number of cubic inches were found in the atmosphere, in chapter IX. of this work. The same author observes, that, for every square inch of the earth's surface, about 41 cubic inches of water are annually evaporated; so that the average quantity of rain is considerably less than the average quantity of water evaporated.

SNOW AND HAIL. Snow consists of such vapours as are frozen while the particles are small; for, if these

* Dr. Rutherford's Natural Philosophy, vol. ii. chap. 10. Signior Beccaria, whose observations on the general state of electricity in the atmosphere have been very accurate and extensive, ascribes the cause of hail, rain, snow, &c. &c. to the effect of a moderate electricity in the atmosphere.

+ Mr. Adam Walker's Familiar Philosophy, lect. v. page 215. Dr. Thompson's Chemistry, vol. iv. p. 83, &c. edition of 1810,

stick together, after they are frozen, the mass that is formed out of them will be of a loose texture, and form little flakes or fleeces, of a white substance, somewhat heavier than the air, and therefore, will descend in a slow and gentle manner through it. Hail, which is a more compact mass of frozen water, consists of such vapours as are united into drops, and are frozen while they are falling.*

7. THUNDER AND LIGHTNING. It has been already observed, that the atmosphere is the common receptacle of all the effluvia, or vapours, rising from different bodies. Now, when the effluvia of sulphureous and nitrous bodiest meet each other in the air, there will be a strong conflict, or fermentation, between them, which will sometimes be so great as to produce fire. Then, if the effluvia be combustible, the fire will run from one part to another, just as the inflammable matter happens to lie. If the inflammable matter be thin and light, it will rise to the upper part of the atmosphere, where it will flash without doing any harm; but if it be dense, it will lie near the surface of the earth, where, taking fire, it will explode with a surprising force, and by its heat rarefy and drive away the air, kill men and cattle, split trees, walls, rocks, &c. and be accompanied with terrible claps of thunder. The effects of thunder and lightning are owing to the sudden and violent agitation the air is put into, together with the force of the explosion. Stones and bricks struck by lightning are often found in a vitrified state. 'Signior Beccaria supposes that some stones in the earth, having been struck in this manner, gave rise to the vulgar opinion of the thunder-bolt. It is now generally admitted that lightning and the electrical fluid are the same.§

*Rutherford's Philosophy, vol. ii. chap. 10.

+ Gunpowder, the effects of which are similar to thunder and lightning, is composed of six parts of nitre, one part of sulphur, and one part of charcoal.

Professor Winkler's Philosophy.

Signior Beccaria, of Turin, observes, that the atmosphere abounds with electricity; and if a cloud which is positively charged (viz. which · has more than its natural share of electrical fluid) pass near another cloud which is negatively charged (viz. which has less than its natural share of electrical fluid) they will attract each other, and a quick depri