MACHINES 



69 



is used to pump water, grind corn, and do other, kinds 

 of work which the farmer has to do. In Holland the 

 water is pumped from the lowlands (Figure 67) by numerous 

 windmills. The sailboat is a contrivance for utilizing 

 the power of the wind for travel and transportation on 



water. The sail of ^___ 



the boat merely 

 provides a large 

 area of resistance 

 to the wind. 



The Aeroplane. 

 There are a 

 number of craft 

 to which the term 

 airship may prop- 

 erly be applied, 

 but the aeroplanes 

 only are properly 

 called flying ma- 

 chines. They are 

 maintained in 

 their positions by 

 the resistance of 

 the air. When 

 the motor is work- 

 ing, the aeroplane 

 is driven rapidly 

 forward by the action of the propeller against the resist- 

 ing air. Then when the plane or planes of the airship are 

 tilted upward, the resistance which the air offers to their 

 forward movement causes the machine to move upward. 

 The aeroplane is heavier than air and must be in rapid 

 forward motion in order to maintain its position in the air. 



FIG. 66. Windmill. 



