The small ragged clouds which are sometimes seen sailing rapidly through the air, are called _scud_. They consist of portions of a rain-cloud, probably broken up by the wind, and are dark or light according as the sun shines upon them. They are the usual harbingers of rain, and, as such, are called by various names, such as _messengers_, _carriers_, and _water-waggons_. * * * * * In attempting to explain the production of clouds and rain, it is necessary to observe that the subject is beset with difficulties—the discussion of which does not belong to this little volume; but the following notice of Dr. Hutton’s theory may not be out of place. It has been already stated, that the air supplies itself with moisture from the surface of the waters of the earth. This it continues to do at all temperatures, until it is so charged with vapour that it cannot contain any more. The air is then said to be _saturated_. Now, the quantity of moisture which a given bulk of air can contain, depends entirely upon the temperature of the air for the time being. The higher the temperature of the air the greater will be the quantity of vapour contained in it; and, although it may be perfectly invisible to the eye, on account of the elasticity which the heat imparts to it, yet it can easily be made visible by subtracting a portion of the heat. If, for example, a glass of cold water be suddenly brought into a warm room, moisture from the air will be condensed upon the outside of the glass in the form of dew. A similar change is supposed to take place when two currents of air having different temperatures, but both saturated with vapour, are mingled together; an excess of vapour is set free, which forms a cloud or falls down as rain. If the currents continue to mingle uniformly, “the clouds soon spread in all directions, so as to occupy the whole horizon; while the additional moisture, incessantly brought by the warmer current, keeps up a constant supply for condensation, and produces a great and continued deposition of moisture in the form of rain. By degrees, the currents completely intermingle, and acquire a uniform temperature; condensation then ceases; the clouds are re-dissolved; and the whole face of nature, after being cooled and refreshed by the necessary rain, is again enlivened by the sunshine, thus rendered still more agreeable by its contrast with the previous gloom.”