by the reflection of the blue light from the air. A similar change of colour is observed on the snowy tops of the Andes and Alps. Between the parts of opaque or coloured bodies, are many spaces, as I have already said, which are either empty or replenished with mediums of other densities. Hence they become transparent, by filling their pores with any substance of an equal, or nearly an equal density with their parts. Thus paper dipped in water or oil; the oculus mundi, or hydrophanous stone, steeped in water; and many other substances, soaked in such liquors as will intimately pervade their pores, become by that means more transparent than otherwise. So on the contrary, the most transparent substances may, by evacuating their pores, or separating their parts, be rendered sufficiently opake, as salts or wet paper, or the oculus mundi, by being dried; horn, by being scraped; glass, by being reduced to powder, or otherwise flawed; and water, by being formed into many small bubbles, either alone in the form of froth, or by shaking it together with oil of turpentine, or some other convenient liquor, with which it will not perfectly incorporate. But, But, I will here release you from this complicated subject. The height of the atmosphere, as I formerly explained to you, has never yet, and never can, in my opinion, be thoroughly ascertained. The elevation of vapours, however, is in some respects, precisely to be known. The height at which vapours freeze, says Buffon, is about 2400 fathoms in the torrid zone, and about 1500 fathoms in France. The highest clouds, continues he, do not rise above the level of the sea more than 3600 fathoms: hence, if our mountains were higher than they are, we should see in the torrid zone a belt of snow commencing at 2400 fathoms above the level of the sea, terminating at 3500, or 3600 fathoms, not on account of the cessation of cold, which augments in proportion to the elevation, but because the vapours would not rise higher. But, I shall have a good deal to say to you on this head, in some future letter; I shall now, therefore, conclude with just remarking, that the vapourous, or obscure part of our atmosphere, is only about the height of the 1980th part of the earth's diameter, as is evident from the height of the clouds, which is never above four miles. LETTER. LETTER XXVI. THE sublime and yet beautiful process of evaporation, which relieves the earth from superfluity, and adorns our horizon with tints of the most glowing brightness, is yet, because not familiar to our senses, of apparently less consequence than that of condensation. The vapours which we have seen to rise, at length precipitate themselves in rain, in snow, or in hail. If on low lands, or on mountains, whose elevations do not reach to the freezing region, they glide away, or rush down in torrents. If, on the contrary, the Cordeliers, or the Alps, are their receptacle, they there concrete into solidity, and at their summer's leisure, but with ponderous dignity stream along the earth, for the benefit of the creation. The larger, as well as smaller rivers, proceed either from a confluence of brooks and rivulets, or from lakes; but no river of considerable magnitude flows from one spring or one lake, but is augmented by the accession of others. Thus Thus the Wolga receives above two hundred rivers and brooks, before it discharges itself into the Caspian sea; and the Danube receives no less, before it enters into the Euxine. Some rivers, those, for instance, in the country where you reside, are greatly augmented by the dissolution of the ice and snow. In the country of Peru and Chili, there are small rivers, it is said, that only flow in the day, because they are only fed by the snow on the mountains of the Andes, which is then melted by the heat of the sun. There are also, it is reported, several rivers upon both sides the extreme parts of Africa, and in India, which, for the same reason, are greater by day than by night. The rivers also in other places are almost dried up in summer, but swell and overflow their banks in winter, or in the wet season. Thus the Wolga, in May and June, is filled with water, and overflows its shelves and islands, though at other times of the year it is so shallow, as scarcely to afford a passage for loaded vessels. The Nile, the Ganges, the Indus, are so much swelled with rain, or melted snow, that they overflow their banks; and these deluges happen at different times of the year, because they proceed from different causes. Those that are swelled with rain, are generally highest in winter, because rain is usually then more fre quent quent than at other times of the year; but if they proceed from ice or snow, which in some places are melted in the spring, in others in the summer, or between both, the deluges of the rivers happen accordingly. The running of rivers, as I have already remarked to you, is upon the same principle as the descent of bodies on inclined planes; for it is as impossible for water, as for asolid body, to move on an horizontal plane; the re-action of such a plane, being equal and contrary to gravity, entirely destroys it, and leaves the body at rest. Here I speak of a plane of small extent, and such as coincides with the curved surface of the earth. But if we consider a large extent, or long course of water, then we shall find that such water can never be at rest, but when the bottom of the channel coincides every where with the curved surface of the earth. Hence, therefore, since bodies move on planes even in the smallest degree inclined, except so far as they are prevented by friction; and since the friction of the particles of water among themselves is inconsiderable, it follows, that the water situated on a plane ever so little inclined, will commence a motion'; and if the plane be considerably inclined, and the quantity of water great, |