138 INTRODUCTION TO GENERAL SCIENCE 



Experiment 53. To Make a Windmill. 



Materials : Piece of paper, eight inches square, pin. 



a. Fold the paper diagonally twice, then tear, or cut, along 

 the diagonals two thirds the distance from the corners to the 

 center. Fasten alternate corners to the center with a pin. 

 This produces a four-blade windmill which will revolve in the 

 current of air produced by a person while walking. Such 

 wind wheels may be made double, or of any number of sheets, 

 and in colored paper be used for purposes of decoration. 



101. HUMIDITY 



In addition to the various gases which compose the atmos- 

 phere, there is a varying quantity of water vapor, which is the 

 most important substance. Without this water vapor there 

 could be no growth of vegetation, for there could be no rain 

 under any circumstances. Water exists in several states; it 

 appears as clouds, fog, mist, rain, dew, frost, snow, hail, and 

 ice. Water vapor is invisible. 



The water vapor makes the air damp, although it does not 

 affect the amount of air which is present. That is, water 

 vapor does not crowd out the air, and, on the other hand, no 

 more water vapor would be present, under given conditions of 

 temperature and pressure, if the air were wholly removed. 



Sulphuric acid has the power of absorbing water, and an 

 open dish of this acid is used to remove the moisture from 

 within clock and balance cases, in order to prevent the rusting 

 of delicate parts. Such a dish of acid, if exposed to the open 

 air, rapidly gains in weight, due to the water which is absorbed 

 from the air. 



The total amount of water vapor which the air can hold 

 under given conditions is called saturated humidity; the 



