20 deg. Fahr. 1.30 grains Troy. 70 deg. Fahr. 8.00 grains Troy. 32 “ “ 2.13 “ “ 80 “ “ 10.95 “ “ 50 “ “ 4.09 “ “ 90 “ “ 14.81 “ “ 62 “ “ 6.15 “ “ 100 “ “ 19.79 “ “ This gives a second reason why storms of wind and rain closely follow extremely hot weather. Now, as the vapor is so much lighter than the air, their mixture must also be lighter. So any unusual amount of moisture is at once detected by the barometer, an instrument for measuring the pressure of the atmosphere. If the air grow moister, and therefore lighter, the barometer falls; a storm is approaching. Since cold air can retain but little moisture, if a warm moist current be chilled, it must lose a part of its vapor, which at once falls to the earth as rain. If the cold be somewhat greater, the moisture is crystallized into snow. Greely’s observations at Fort Conger show that, varied as are the forms of snow crystals, those that fall during any particular storm are invariably of the same types, even though they may be collected from localities widely removed from each other. All crystals of snow are hexagonal in plan, but there is much variety in detail. The laws that produce one variety at one time, and a second at another, are not yet known. The subject of hail is a peculiarly perplexing one to the meteorologist. Hailstones are more or less spherical in form, and are made of alternate layers of soft opaque ice, and hard clear ice. It is evident that they must acquire this structure by being whirled about between clouds of different temperature and density. Some have supposed that they are formed in a whirlwind, whose axis is horizontal, but for the present we must be content with Lord Dundreary’s explanation, for “it ith one of thothe thingth which no fellah can underthtand.” Raindrops from a great height are larger than those from below, for they increase as they pass through the vapor-masses. As the warmest currents are also the highest, it will at once be understood why warm and tropical rains fall in large drops, while drizzling rains, mists, and fogs are characteristic of cold regions and cold seasons. The masses of more or less condensed vapor in the upper air currents are what are known as clouds. Their various forms and appearance are shown in the cut on the ---- page. The _cirrus_ and _cirro-cumulus_ clouds are the highest, are mostly in the altitudes of perpetual frost, and are supposed often to consist of minute ice crystals. In temperate latitudes they are usually formed in, and move with, the upper air current, or return-trade from the tropical regions.