72 



LOVES MEINIE. 



you, put in ver}'- small space ; with these only 

 differences : in a boat, the air strikes the sail ; 

 in a bird, the sail strikes the air : in a boat. 



Fig. 3. 



a 



the force is lateral, and in a bird downwards ; 

 and it has its sail on both sides. I shall leave 

 you to follow out the mechanical problem for 



