28 THE POPULAR SCIENCE MONTHLY 



Let us come down from the highest atmosphere to some of the 

 phenomena nearer the earth's surface. Possibly yon 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 j^ears 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 tem- 

 perature 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 filled 

 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 



