rain is greater in summer than in winter. More rain fails 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. Thomson 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. 6. SNOW AND HAIL. Snow consists of such vapours as are frozen while the particles are small; for, if these 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 * Rutherford's Philosophy, vol. iii. 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. 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 thunderbolt. It is now generally admitted that lightning and the electrical fluid are the same.* 8. THE FALLING STARS, and other fiery meteors, which are frequently seen at a considerable height in the atmosphere, and which have received different names according to the variety of their figure and size, arise from the fermentation of the effluvia of acid and alkaline bodies, which float in the atmosphere. When the more subtle parts of the effluvia are burnt away, the viscous and earthy parts become too heavy for the air to support, and by their gravity fall to the earth. 9. OF THE IGNIS FATUUS, commonly called Willwith-a-Wisp, or Jack-with-a-Lantern. This meteor, like most others, has not failed to attract the attention of philosophical inquirers. Sir Isaac Newton, in his Optical Queries, calls it a vapour shining without heat. Various accounts of it may be seen in the Philosophical Transactions. The most probable opinion is, that * 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 Auid) they will attract each other, and a quick deprivation of the electrical fluid will take place : the flash is called lightning, the report thunder, (the ensuing rolling are only echoes from distant clouds): the water, thus deprived of its usual support, falls down in impetuous torrents. Mr. Ray and some others supposed it to be a collection of glow-worms flying together; but Dr. Derham confuted this opinion, No. 411. it consists of inflammable air,* or oleaginous matter, emitted from a putrefaction and decomposition of vegetable substances, in marshy grounds; which being kindled by some electric spark, or other cause unknown to us, will continue to burn or reflect a kind of thin flame in the dark, without any sensible degree of heat, till the matter which composes the vapour is consumed. This meteor never appears on elevated grounds, because, they do not sufficiently abound with moisture to produce the inflammable air, which is supposed to issue from bogs and marshy places. It is often observed flying by the sides of hedges, or following the course of rivers; the reason of which is obvious, for the current of air is greater in these places than elsewhere. These meteors are very common in Italy and in Spain. Dr. Shawt has described a remarkable ignis fatuus, which he saw in the Holy Land, when the atmosphere was so uncommonly thick and hazy, that the dew on the horses' bridles was remarkably clammy and unctuous. This meteor was sometimes globular, then in the form of the flame of a candle, presently afterwards it spread itself so much as to involve the whole company in a pale harmless light, and then it would contract itself again, and suddenly disappear; but, in less than a minute, it would become visible as before, and running along from one place to another with a swift progressive motion, would again expand itself, and cover a considerable space of ground. 10. OF THE AURORA BOREALIS, or NORTHERN LIGHTS. There have been various opinions and conjectures respecting the cause and properties of these extraordinary phenomena; and the most probable opinion is, that they arise from exhalations, and are produced by a combustion of inflammable air, caused * Inflammable air may be made thus: exhaust a receiver of the air-pump, let the air run into it through the flame of the oil of turpentine, then remove the cover of the receiver, and hold a lighted candle to the air, it will take fire, and burn quicker or slower according to the density of the oleaginous vapour. † Shaw's Travels, page 363. Philosophical Transactions, Nos. 305, 310, 320, 347, 348, 349, 351, 352, 363, 365, 368, 376, 385, 395, 398, 399, 402, 410, 418, 431, and 433, &c. by electricity. This inflammable air is generated particularly between the tropics, by many natural operations, such as the putrefaction of animal and vegetable substances, volcanoes, &c.; and being lighter than any other, ascends to the upper regions of the atmosphere, and, by the motion of the earth, is urged towards the poles; for it has been proved by experiments that, whatever is lighter, or swims on a fluid which revolves on an axis, is urged towards the extreme points of that axis:* Hence, these inflammable parti cles continually accumulate at the poles, and by meeting with heterogeneous matter, take fire, and cause those luminous appearances frequently seen towards the polar regions.t In high latitudes the Aurora Boreales appear with the greatest lustre, and extend over the greatest part of the hemisphere, varying their colours from all the tints of yellow to the most obscure russet. In the north east parts of Siberia, Hudson's Bay, &c. they are attended by a continued hissing and cracking noise through the air, similar to that produced by fire works.§ 11. OF THE RAINBOW. The rainbow is the most beautiful meteor with which we are acquainted: it is never seen but in rainy weather, where the sun illuminates the falling rain, and when the spectator turns his * See Mr. Kirwan's account of the Aurora Borealis, Irish Phil. Transactions for 1788, page 70, &c. We have very few accounts of the Aurora Australis, or Southern Lights, owing perhaps to the want of observations in those remote parts of the globe, and a proper channel of information. Captain Cook, in his second voyage towards the south pole, says: "(February 17th, 1773) We observed a beautiful phenomenon in the heavens, consisting of long columns of clear white light, shooting up from the heavens to the eastward, almost to the zenith, and gradually spreading over the whole southern part of the sky. Though these columns were in most respects similar to the Aurora Borealis, yet they seemed to differ from them in being always of a whitish colour. The stars were sometimes hid by, and sometimes faintly to be seen through, the substance of these Aurora Australes. The sky was generally clear when they appeared, and the air sharp and cold, the thermometer standing at the freezing point; the ship being in latitude 58° south." Dr. Rees' New Cyclopædia, word Aurora Borealis. back to the sun. There are frequently two bows seen, the interior and exterior bow. The interior bow is the brightest, being formed by the rays of light falling on the upper parts of the drops of rain; for a ray of light entering the upper part of a drop of rain will, by refraction, be thrown upon the inner part of the spherical surface of that drop, whence it will be reflected to the lower part of the drop, where, undergoing a second refraction, it will be bent towards the eye of the spectator; hence, the rays which fall upon the interior bow come to the eye after two refractions and one reflection, and the colours of this bow from the upper part are red, orange, yellow, green, blue, indigo, and violet. The exterior bow is formed by the rays of light falling on the lower parts of the drops of rain ; these rays, like the former, undergo two refractions, viz. one when they enter the drops, and another when they emerge from the drops to the eye; but they suffer two or more reflections in the interior surface of the drops; hence, the colours of these rays are not so strong and well defined as those in the interior bow, and appear in an inverted order, viz. from the under part they are red, orange, yellow, green, blue, indigo, and violet. To illustrate this by experiment, suspend a glass globe filled with water in the sun-shine, turn your back to the sun, and view the globe at such a distance that the part of it the farthest from the sun may appear of a full red colour, then will the rays which come from the globe to the eye make an angle of 42 degrees with the sun's direct rays; and if the eye remain in the same position, and another person lower the glass globe gradually, the orange, yellow, green, &c. colours, will appear in succession, as in the interior bow. Again, if the glass globe be elevated, so that the side nearest to the sun may appear red, the rays which come from the globe to the eye will make an angle of about 50 degrees; then, if another person gradually raise the glass globe, while the spectator remains in the same position, the rays will successively change from red to orange, green, yellow, &c. as in the exterior bow. These observations being understood, let dne (Plate IV. Fig. 1.) represent a drop of rain belonging to the interior bow, Sd a ray of light falling on the upper part |