The _cumulus_ clouds are characteristic of the tropics, and of the summer days in middle latitudes, their height depending upon the relative humidity of the air. They are formed by local ascending currents, which carry a large amount of vapor into the cooler upper air. There the vapors are condensed, and are gradually heaped up into those heavy masses of sharply defined clouds, which look like vast snowy mountains. Their base is horizontal, and marks the height at which the dew point is reached and condensation begins. The accumulation of vapors is often so great that these clouds form a column several thousand feet high. In this case the difference in the temperature and the electrical conditions of the upper and lower portions is such that electrical discharges take place, accompanied by condensation of a portion of the cloud, forming a thunderstorm. _Stratus_ clouds are most frequently seen in the morning or evening, and are always low. They are formed by the descent of the higher clouds and vapors of midday into the lower air as the temperature decreases. They are more frequent in winter and summer than in the intermediate seasons. The _nimbus_ cloud is more dense and heavy than the others, which may all be transformed into the nimbus by [Illustration: FORMS OF CLOUDS.] a diminution of temperature. It is of a dark-leaden hue, changing into grey. This is the most common form of cloud in polar latitudes; and, during the cold season, it is the most frequent of the temperate zones. If a moist current cross a mountain range, it loses its moisture in the cold region, and growing narrower as it descends the other slope, presents the phenomena of a warm dry wind from the mountains. Thus the wind that brings rain to Norway, gives warm fair weather to Sweden. Of the same character are the hot winds of Switzerland, called Foëhn winds, and the Chinook winds which blow from the eastward into Idaho, Washington, and western Montana. Similar winds occur occasionally in South Africa, Australia, New Zealand, and Peru. These hot winds must not be confounded with the hot and poisonous winds from desert regions described before. Such, in fine, are the noted varying intermittent, or periodic winds. However uncertain they may appear at first thought, they are obedient to the same unchanging laws that bind the universe into one harmonious whole. No doubt the ancients, if they had been acquainted with their office, would have personified them as the nymphs of the seasons. But, knowing naught of the wonderful immutable laws that bind them, they could only say to each, “We know not whence thou com’st, or whither goest, When round our homes thy wizard blast thou blowest.” In eternal law and harmony they found only endless confusion and wild caprice. Man interpreted nature by man.