The two prime causes of rain are, of course, the sun and the ocean; and since these two factors do not appreciably vary from year to year, it follows that the annual rainfall on the earth as a whole, if it could be measured, would also be found to be invariable. It is obvious, however, that the rainfall at all places is not equal. In London, for instance, the average yearly rainfall is twenty-two inches; but on the Khasi Hills in India it is no less than six hundred inches. Similar contrasts are observable in other parts of the world, the differences being due to local geographical conditions. The starting points in the history of rain are, therefore, heat and moisture. From the surface of land and water tiny globules or vesicles of moisture are continually rising into the atmosphere by the force of the sun’s heat; and the warmer the air the greater the number of these globules of water the atmosphere is able to absorb. In this respect the atmosphere may be likened to a sponge, for it is from the moisture thus retained that the subsequent raindrops are formed. Most persons are well acquainted with the very familiar phenomenon which is to be noticed when a glass of very cold water is brought into a warm room: the drops of moisture which form on the outside of the glass being among the commonest phenomena in what may be termed domestic meteorology. There is a similar transformation in the outside atmosphere; so that when the warm, moist currents of air flow against the sides of a cold mountain, or it may be against a body of cold air, there is a reduction in temperature, the atmosphere is squeezed like a sponge, and the particles of moisture are forced out of it. The particles then assume the form of cloud, fog, mist, rain, snow, and hail, as the case may be. Now, as regards the globules of moisture, the most recent experiments and observations point to the conclusion that before the drops of vapor can form, there must be a tiny nucleus of dust upon which the condensed water may settle. At the centre of every drop of vapor in a cloud there is probably a little core of dust; and without these little atoms there could be no rain. These atoms of dust are visible only under the strongest microscopes; and so minute are they that in a cubic foot of saturated air it has been calculated that they number one thousand millions, their total weight being only three grains.