Transportation in the Air 



397 



indicate the magnitude of a given wind pressure. Measure on the 

 wind pressure line the right distance, marking the end with the point 

 of an arrow. What is the length of a line to represent 50 grams? 

 40 grams? 10 grams? 



From the point you have taken as representing the center of the 

 kite, draw a vertical line to show the direction of the " lift." To find 



Fio. 139. Diagram to show why a kite flies. Make your diagram on a scale 

 of 1 cm. to 10 grams. 



the magnitude of the lifting force, draw from the end of the wind 

 pressure line a line perpendicular to the vertical line. Measure the 

 length of the vertical line to the point of intersection, and compute 

 the magnitude by use of the scale number. What force would be 

 represented by a line 4 centimeters long? 12 centimeters? 15 centi- 

 meters ? 



Reference to the problem of sailing a boat into the wind and the 

 parallelogram of forces will be helpful to you in the following prob- 

 lems. In what position will the kite be when the lifting force is most 

 nearly equal to the wind pressure? Show the magnitude of the 

 drift or pushing force as you have that of the lifting force. 



What must be the angle of the kite to the wind, in order that the 

 lift and drift be equal ? If this is not at once evident, it may be deter- 

 mined by experimental drawing with the kite placed at different 

 angles. 



Can a kite be made to fly with the string exactly vertical ? If the 

 string breaks, and the kite is allowed to move with the wind, what 

 happens to the kite? If there is no wind, can a kite be made to fly? 



