710 CLOUDS particles were globules of water ; but Sir J. Herschel offers the explanation that the globules may be as minute as the lengths of light undu- lations, and would not therefore, by refraction or reflection, form the solar spectrum. It ap- pears, moreover, that as long ago as 1847 (" Philosophical Transactions"), Dr. A. Waller disproved the existence of vesicles by project- ing steam upon the surface of Canada balsam and examining the particles with a microscope, when it was found that they were not hollow. Assuming this to be a fact, the suspension of clouds in the air is not difficult of explanation when we reflect that, as Prof. Stokes has de- monstrated, a globule of water - t J fl of an inch in diameter falls through the air with a ve- locity of only -067 of an inch per second, a motion which is inappreciable when compared to the upward movement which the air gene- rally has in large clouds. The various forms in which clouds present themselves are de- scribed after the nomenclature introduced by Mr. Luke Howard in the " Askesian Lectures," 1802 (published in vols. xvi. and xvii. of the "Philosophical Magazine"). He recognizes FIG. 1. Cumulus. three primary modifications, the cumulus, stra- tus, and cirrus ; and intermediate between these, into which they blend, three other forms, the cumulo-stratus, cirro-stratus, and cirro-cumu- lus ; and lastly, the nimbus, resulting from the others confusedly intermixed. The cumulus is the summer day cloud. About sunrise it is seen to collect by the gathering of the small specks of cloud, which are the scattered night mists, and which, accumulating, present the ap- pearance of distant rounded hills covered with snow. In calm weather it is obviously formed from the columns of vapor that rise irregularly from the surface, invisible below, but brought into view above, as the particles reach the stra- tum of the atmosphere already at its dew point. There the vapors, arrested in their ascent, form piles of clouds of hemispherical shapes, the apparently flat bases of which mark the level where hygrometric saturation commences. As with the increasing warmth of the day more vapor is carried upward, these piles of clouds increase in height and density. They obscure the rays of the sun, by which, however, they may be dispersed before the decline of day, or they may be gathered and condensed by cool winds, preparatory to returning their moisture to the earth in the form of rain. The stratus is the cloud of night and of winter, though not limited to these periods any more than the cumulus is to that of the day. As the day cloud is produced by the ascending vapors, that of the night, called also the fall cloud, is, excepting fogs formed by exhalation, the result of their descent, their settling down in hori- zontal layers or strata. This form of cloud is FIG. 2. Stratus. at times suddenly produced, overspreading the heavens in a few minutes, by the temperature of the air falling by radiation or by diminished atmospheric pressure. It falls to a lower level than other clouds, creeping along the valleys at night in the form of mists and fog, and van- ishing with the return of day. In winter it continues as an overhanging cloud sometimes for several successive days. The smoke fogs are one form of the stratus ; at night settling down to a lower level, as if each smoky parti- cle by radiation of heat collected dew, and sunk by increase of weight. The cirrus is a cloud FIG. 8. Cirrus. of feathery form, and in wisps of diverging fibres. It extends in long slender filaments, and again in parallel stripes from one extremity of the heavens to the other. Its appearance often in flexuous fibres has caused the names of curl cloud and mare's tail to be applied to it. No clouds are seen so elevated as the cirri. From summits where one looks down upon the piles of cumuli, the fleecy cirri are seen floating as far off apparently in the blue ether as when
