and very dry air such as frequently occurs in Colorado, Australia, and India. Under these circumstances all the condensation occurs in the cold layer, and none in the lower, hot one. Portions of the upper layer drop down in large bulbous masses like water balloons, which burst like soap bubbles and drop their moisture like water running out of a cask. Sometimes this water never reaches the ground, being re-evaporated while passing through the hot air.

CHAPTER VIII.

THE RAIN, SNOW, AND HAIL OF THE
ATMOSPHERE.

RAIN is the final stage of condensation of vapour back into water, of which cloud is a halfway stage. The mist which composes a cloud is formed of tiny drops of water about 6-inch in diameter. It used to be a puzzle to explain how these water particles were sustained, and it was at one time supposed that cloud particles were hollow. We know now that this is neither necessary nor true, since very small particles even of gold will remain suspended for a long time in air; the finer the particles the longer they take to fall. A slight upward motion of the air is therefore enough to keep them balanced. As condensation proceeds these particles grow larger by fresh coatings of water, and the larger ones fall down against the smaller and mingle with them until large drops from to inch thick form, which are no longer capable of being suspended and fali

to the earth. Snow forms when the temperature at which this further stage of condensation occurs is below freezing point. Every snow crystal is a variety of a six-rayed cluster, and is similar to the crystals of salts which are precipitated from a chemical solution. No one has watched the formation of snow, but it must be very similar to that of crystallisation out of a solution which is saturated with a chemical salt.

Hail, unlike the delicate snow crystals, is frozen water-drops. Its frequent association with thunder-storms led to the belief that it was caused in some way by electricity. This is, however, found to be untenable in the searchlight of modern science, which shews that electricity is mostly an effect, not a cause of such mechanical disturbances. It is believed, that in such storms the rain-drops formed in one part of a storm are carried upwards by powerful ascending currents (twenty-five miles an hour is enough to sustain large drops) into higher regions of the atmosphere where they are solidified by the excessive cold, and being carried over with the overflow which takes place near the top, fall down until they are redrawn into the interior of the storm and again whirled up aloft. Receiving alternate meltings and freezings, and growing larger with each circuit they make in the atmospheric churn, they are finally thrown out on either side of the storm centre. This explains the fact that in a travelling hailstorm there are two bands where hail falls on either side, while, under the centre, it is often found that only rain has fallen.

Hailstones have often fallen of enormous sizes. In 1697, Robert Taylor found hailstones in Hertfordshire 14 inches in circumference.

In India, the writer remembers a hailstorm on the great Brahmaputra river when the hailstones cut holes through the tarpaulin cover of the steamer, which were so large that each one had

to be mended with a separate patch. Hailstorms are intimately connected with tornadoes, and, like these phenomena, are more frequent over flat plains and in very hot and moist summers and countries.

The destruction dealt by hail on standing crops, vineyards, and orchards has led to means

being proposed for its prevention. Under the old idea of its connection with electricity, lightning rods were erected, but without avail. With ironical waywardness it fell in several instances on the rod-protected lands and avoided the others.

Planting of trees would be more effective, since this would tend to check the rapid heating up of the lowest stratum of air which is one of the chief causes of tornado and hailstorm action.

The general distribution of rain in belts over different areas of the earth's surface has already been alluded to.

Rainfall, like clouds, is more prevalent in mountainous than over flat countries, and for similar reasons, especially cooling by forced ascent of air.

In the accompanying mean annual rainfall maps of England and India, this will be readily seen. In England the heaviest falls will be observed to occur in the mountains of Cumberland and Wales, and generally along the hilly country of the West and North. In Scotland and Ireland

it is the same. The lowest rainfalls under 20

inches all occur on the eastern sides of the country. This difference is partly due to the fact that the prevailing and most rainy winds are southwest and drop a good deal of their moisture before reaching the eastern parts, but even were these barriers absent, the rainfall over the flatter country on the eastern sides would not be very much increased. In India, in like manner the dark shading along the Western Ghats down the Bombay coast and along the Himalaya shews the influence of the mountains, the heaviest fall occurring near the north-east corner of the Bay of Bengal in the Khasià hills, which offer an abrupt

wall 4000 feet high up which the southerly monsoon winds, see fig (20), are forced.

Chirapunji, at the edge of these hills, has the largest rainfall in the world (about 500 inches),

half of which falls in June and July.

On the western side of the Ghats rain falls heavily up to 250 inches at Mahalleshwar on their summits, while the tableland of the Deccan on their eastern lee side has a scanty supply and is one of the areas liable to drought.

The greatest amount of rain in a vertical direction occurs at altitudes where the lowest