Page:Popular Science Monthly Volume 78.djvu/32

28 Let us come down from the highest atmosphere to some of the phenomena nearer the earth's surface. Possibly you may think that to the agriculturists the vital question is how to make it rain or how to stop the rain, according to the needs of the farmer. You may ask what are the ultimate causes of calamitous droughts, such as those of Syria, India and Australia, or the less injurious dry periods in Europe and America. These usually result from several successive years of deficient rainfall, as in the famous Biblical story of seven years of high water and seven years of low water, in the river Nile, in the days of Joseph. We have now many years of continuous record of the fluctuations of this great river and we know something of its irregularities. In order to understand why and when these droughts should occur, we must first understand how rain and snow are formed in the clouds and why rain does not always fall from the clouds. I have here on this laboratory table a small globe filled with the vapor of water mixed with air as it ordinarily occurs in the atmosphere. Now we know that when moist air rises up to the level of the clouds, it has expanded and by pushing aside the adjacent air has done work in its expansion. That work has used up some of the internal energy of the air which we call heat energy, so that the air has become cooler, just as steam expands and pushes the piston of a steam engine. When by this cooling the temperature of the moist air has been so reduced that it is near the dew-point, then the air is saturated with moisture and a cloudy condensation begins. This invisible vapor in the air begins to condense around every little particle of dust and every invisible electron. You have seen an ice pitcher covered with moisture on a warm summer day. In the same way these atmospheric dust particles are covered with moisture. I will now allow the air in this globe to expand by opening this lower stopcock leading to a low pressure chamber and you will notice the formation of a slight cloud of haze. The cloud is, however, not very dense because there is not much dust in the air. I will now repeat the experiment. First I will exhaust the air already in the globe, then close the lower stopcock. I wish to introduce into the globe more dust than is in the ordinary air of the room. To do so I light a match and hold it so that the smoke from the flame is near the upper end of the tube. When I turn the upper stopcock so that the vacuous space may become fllled with air, the inrushing air carries the smoke in with it. I close the upper stopcock and now the globe is full of dusty moist air. I open the lower stopcock, this dusty air expands downward into the lower pressure chamber and you see a dense cloud of fog is formed. These successive steps illustrate the ordinary method of the formation of clouds, but not of rain. To understand that, we must go a step further. Thus far I have allowed the dusty moist air expanding downward to increase its volume in the ratio of 600 or 650 to 760, i. e., 1 to 1.2 or 1.3. I will