94 GENERAL SCIENCE 



The cloud masses consist of minute droplets of water which are con- 

 tinuously settling toward the earth. While in a general way their 

 forms persist, clouds are all the time undergoing change by wasting 

 at the lower surfaces and by reforming higher up. 



When the temperature of the air falls below freezing, the water 

 vapor it contains may be changed to frost crystals. Masses of these 

 crystals form snowflakes. When the ground, the roofs of buildings, 

 and other surfaces are below freezing temperature, and the air is 

 warmer and moisture-laden, frost is likely to form upon the cold 

 surfaces. 



The presence of water in the air, whether as vapor or as cloud par- 

 ticles, serves more or less as a blanket, and lessens the rate of xrooling 

 of the earth's surface at night. 



As the air cools at night its capacity for holding moisture decreases. 

 With the same amount of vapor of water present in the air, its relative 

 humidity or per cent of saturation increases. In the early evening 

 this increase is often rapid. 



Condensation is the opposite of vaporization, and both processes 

 are involved in distillation. Natural distillation, with the sun 

 as source of the heat, plays a large and important part in weather con- 

 ditions. The rainfall of any region that is destitute of large water 

 surfaces is dependent in great part upon winds blowing from other 

 regions where vaporization is abundant. These winds, if unduly con- 

 tinued, result in a wet season for inland areas, while their deflection or 

 cessation for causes unknown accounts for a dry season. 



HEAT OF VAPORIZATION, AND OF FUSION 



It has been found that to change the temperature of one 

 pound of water one degree on the Fahrenheit thermometer 

 always involves the same amount of heat. It makes no 

 difference whether the water is warmed or cooled. This 

 quantity of heat is used as the unit in computation of heat 

 values, and is known as the British thermal unit. Then 

 if water weighing twelve ounces (three-fourths pound) has its 

 temperature raised 6J^ F., the water has gained % X 6^, 

 or 4% British thermal units. 



Most science texts make use of the metric system of 

 weights and measures. It is much more simple to learn 



