134 THE REIGN OF LAW. 



The Law of Gravitation, therefore, is used in the 

 flight of birds as one of the most essential of the 

 Forces which are available for the accomplishment 

 of the end in view. 



The next law appealed to, and pressed into the 

 service, is again a law which would seem an im- 

 pediment in the way. This is the resisting force 

 of the atmosphere in opposing any body moving 

 through it. In this force an agent is sought and 

 found for supplying the requisite balance to the 

 Force of Gravity. But in order that the resisting 

 force of air should be effectual for this purpose, it 

 must be used under very peculiar conditions. The 

 resisting force of fluids, and of airs or gases, is a 

 force acting equally in all directions, unless special 

 means are taken to give it predominant action in 

 some special direction. If it is a force strong 

 enough to prevent a body from falling, it is also a 

 force strong enough to prevent it from advancing. 

 In order, therefore, to solve the problem of flight, 

 the resisting power of the air must be called into 

 action as strongly as possible in the direction 

 opposite to the Force of Gravity, and as little 

 as possible in any other. Consequently a body 



