THE CONQUEST OF THE AIR 81 



Thus knowing the strength and direction of the pull of the two 

 combined forces, the resultant may be determined; or knowing 

 the resultant and the direction of the pull of the component forces, 

 the latter may be determined. 



Forces acting along DE and DG in unison on point D in the 

 direction of the points C and A combine to produce the effect 

 of a force acting along line DF which is counteracted by the equili- 

 brant pull on the scale at B along line ED. As calculated above, 

 the magnitude of the operating forces and the resultant effect are 

 in proportion to the sides of the parallelogram and to the diagonal. 



Conversely, suppose a particle of air is moving swiftly along 

 line ED from E toward D y and at D it strikes the surface of the 

 kite KI. It hits the kite a glancing blow at D, and flies off along 

 DE. But the force of the blow at D is resolved into two factors, 

 one of which acting along DG lifts the kite. Successive air 

 particles as the wind blows hit repeated blows. The combined 

 effect sends the kite up. If the length DF represents the inten- 

 sity of the force striking Z), it will be resolved into two com- 

 ponents acting along DE and DG proportional to the lengths of 

 these lines, and the sum of the components will, of course, be 

 equal to DF. The wind is resolved into such factors only when 

 the string holds the kite against the wind. 



The kite mounts in the air as the string is played out until the 

 down pull on the string and the weight of the kite equal the factor 

 DG. If the bridle is so adjusted that the kite lies at an acute 

 angle to the direction of the wind, the factor DG will be small, 

 the kite will fly almost directly overhead, and the pull on the 

 string will be slight. If, however, the bridle is adjusted so the 

 kite makes a relatively large angle to the wind, the factor GD 

 will be great. The kite will pull hard on the string and will 

 sail off to a distance, but will not mount very high. It is evident 

 that in a light breeze the bridle must be adjusted as in the second 

 case to get the kite to go up at all, for it will need much of the 

 force of the wind to raise the kite and its attached string. But 

 in a good stiff breeze the attachment indicated in the first case 



