I. "YOUR buoyant troops on dimpling ocean tread, Wafting the moist air from his oozy bed, AQUATIC NYMPHS!--YOU lead with viewless march The winged vapours up the aerial arch, 15 On each broad cloud a thousand sails expand, And steer the shadowy treasure o'er the land, Through vernal skies the gathering drops diffuse, Plunge in soft rains, or sink in silver dews.-- YOUR lucid bands condense with fingers chill 20 The blue mist hovering round the gelid hill; In clay-form'd beds the trickling streams collect, Strain through white sands, through pebbly veins direct; Or point in rifted rocks their dubious way, And in each bubbling fountain rise to day. [_The winged vapours_. l. 14. See additional note No. XXV. on evaporation.] [_On each broad cloud_. l. 15. The clouds consist of condensed vapour, the particles of which are too small separately to overcome the tenacity of the air, and which therefore do not descend. They are in such small spheres as to repel each other, that is, they are applied to each other by such very small surfaces, that the attraction of the particles of each drop to its own centre is greater than its attraction to the surface of the drop in its vicinity; every one has observed with what difficulty small spherules of quicksilver can be made to unite, owing to the same cause; and it is common to see on riding through shallow water on a clear day, numbers of very small spheres of water as they are thrown from the horses feet run along the surface for many yards before they again unite with it. In many cases these spherules of water, which compose clouds, are kept from uniting by a surplus of electric fluid; and fall in violent showers as soon as that is withdrawn from them, as in thunder storms. See note on Canto I. l. 553. If in this state a cloud becomes frozen, it is torn to pieces in its descent by the friction of the air, and falls in white flakes of snow. Or these flakes are rounded by being rubbed together by the winds, and by having their angles thawed off by the warmer air beneath as they descend; and part of the water produced by these angles thus dissolved is absorbed into the body of the hailstone, as may be seen by holding a lump of snow over a candle, and there becomes frozen into ice by the quantity of cold which the hailstone possesses beneath the freezing point, or which is produced by its quick evaporation in falling; and thus hailstones are often found of greater or less density according as they consist of a greater portion of snow or ice. If hailstones consisted of the large drops of showers frozen in their descent, they would consist of pure transparent ice.