Let us extend our observations a little further. The clothes 

 upon the line on wash day are hung out wet and brought in dry. 

 If the sun is shining they probably dry quickly ; but will they 

 not dry even if the sun is not shining? They will indeed ; so here 

 is another fact to add to our other two, namely, (3) that the 

 production of vapor from water will proceed even when the 

 water is not heated. 



This change of water to vapor is called evaporation. The 

 water evaporates from the clothes ; it also evaporates from the 

 walks after a rain, from the mud of the road, from the brooks, 

 creeks and rivers, and from ponds, lakes, and the great ocean 

 itself. Indeed, wherever water is exposed to the air some 

 evaporation is taking place. But heat aids evaporation, as you 

 can prove by taking three dishes of the same kind and pouring 

 the same amount of water into each, then placing one on the 

 stove, a second in the sun and a third in a cool, shady place, as 

 a cellar, and watching to see which is the last to become dry. 



About three-fourths of the earth's surface is covered by 

 water so that the air is receiving vapor all the time. In fact, 

 every minute thousands of barrels of water vapor are rising into 

 the atmosphere from the surface of the ocean. The air is constantly 

 moving about, forming winds, and this load of vapor is there- 

 fore drifted about by the winds, so that the air you are breathing 

 may have in it vapor that came from the ocean hundreds or 

 even thousands of miles away. You do not see the vapor, you 

 are perhaps not even aware that it is there ; but in a room 

 10 feet high and 20 feet square there is often enough vapor to fill 

 a two-quart measure if it could all be changed back to water. 



There is a difference in the amount of vapor from time to time. 

 Some days the air is quite free from it, and then clothes will dry 

 rapidly ; but on other days the air is damp and humid. Then 

 people say it is " muggy " or that the "humidity is high." On 

 these muggy days in summer the air is oppressive because there 

 is so much vapor in it. Near the sea, where there is so much 

 water to evaporate, the air is commonly more humid or moist 

 than in the interior, away from the sea, where there is less water 

 to evaporate. 



We have seen that there is some vapor in all air, but that 



