The International Cloud Classification, now generally used by meteorologists, is an amplification of one introduced by an ingenious English Quaker, Luke Howard, in the year 1803. Howard distinguished seven types of cloud, to which he gave the Latin names _cirrus_, _cumulus_, _stratus_, _cirro-cumulus_, _cirro-stratus_, _cumulo-stratus_, and _nimbus_. In passing, it may be of interest to note that, a few years after Howard's classification was published, an attempt was made by one Thomas Forster to introduce "popular" equivalents of these terms. Forster proposed to call cirrus "curlcloud," cumulus "stackencloud," stratus "fallcloud," etc. In other words, he assumed that because Howard's names were Latin in form they were unsuitable for use by the layman, and therefore needed to be supplemented by English names--although the proposed substitutes were, on the whole, somewhat longer and more difficult to pronounce than the originals! A parallel undertaking would be an attempt to discourage the public from calling the wind-flower "anemone," or virgin's bower "clematis." Forster's superfluous names have never taken root in our language. The highest clouds--cirrus and cirro-stratus--are feathery in appearance, and consist of minute crystals of ice. Their altitude above sea-level averages about five miles, but is frequently much greater than this. All other clouds are composed of little drops of water--not hollow vesicles of water, as was once supposed. Neither crystals nor drops actually "float" in the air. They are constantly falling with respect to the air around them, though, as the air itself often has an upward movement, the cloud particles are not always falling with reference to the earth. In any case, their rate of fall depends upon their size, and in the case of the smaller particles is very slow. Under some conditions the particles evaporate before reaching the earth, while under others they maintain a solid or liquid form and constitute rain or snow. A fog is a cloud lying at the earth's surface. Rainfall is one of the most important elements of climate, chiefly because of its effects upon vegetation. It is measured in terms of the depth of water that would lie on the ground if none of it ran off, soaked in, or evaporated; and this is, in practice, determined by collecting the rain, as it falls, in a suitable receiver, or rain-gauge. Usually the gauge is so shaped as to magnify the actual depth of rainfall, in order to facilitate measurement. Snow is measured in two ways; first, as snow, and, second, in terms of its "water equivalent." The latter measurement is commonly effected by melting the snow and pouring it into the rain-gauge, where it is measured as rain. By this expedient we are enabled to combine measurements of rain and snow, in order to get the total "precipitation" of a place during a given period.