cumulo-nimbus, as they are called. The lower limit of the cloud region is determined therefore by the height at which the rising currents reach their dew-point, and the altitude of the cloud formation depends upon the humidity of the ascending current, the drier it is, so much the higher must it rise to have its vapour condensed. In storms the rising current mingles with the stronger horizontal current above, which carries with it the upper portion of the cloud, and covers the whole sky with a uniform sheet. The wave, or ripple cloud, has been explained by von Helmholtz and von Bezold to be due to the undulations in a horizontal current producing alternate rarefaction and condensation of its water-vapour through changes of temperature. Still another cause of low-lying clouds is the cooling of the air to its dew-point by contact with a cold surface, such as the earth when cooled by radiation during a clear night, or the polar currents of the ocean. Fog is often formed in this way, which we call stratus cloud when it rises above us. The highest clouds consist of ice crystals, because the temperature of the air where they are is much below that of freezing water. Although it is possible to cool drops of water considerably below 32∞ Fahrenheit without congelation, yet it can be told with certainty that the clouds are composed of ice if the sun and moon when seen through them are surrounded by the large rings or halos, which the theory of optics shows can only result from refraction of light by ice crystals, whereas water drops in the clouds produce the smaller coloured rings, which are called coronÊ. The old question, why clouds float unless their particles are hollow, is easily answered, for they do not float, and always tend to sink if they are not supported by the currents of air. In sinking into warmer air the particles are vapourized and become invisible, but others rising are condensed and take their places, so that the cloud persists, although its particles change. This is illustrated by the "cloud banners," which frequently stream from mountain peaks, and are caused by the rise of air up the mountain side. Even in a strong wind the cloud remains attached to the peak, showing that its particles are being renewed continually; but if, as is often the case, the wind descends on the leeward side of the mountain, the cloud particles disappear.