CONTRIVANCE A NECESSITY. 13- 



drawing down all bodies to the earth, is the very 

 Force which is the principal one concerned in 

 flight, and without which flight would be impos- 

 sible. It is curious how completely this has been 

 forgotten in almost all human attempts to navi- 

 gate the air. Birds are not lighter than the air, 

 but immensely heavier. If they were lighter than 

 the air they might float, but they could not fly. 

 This is the difference between a bird and a balloon. 

 A balloon rises because it is lighter than the air, 

 and floats upon it. Consequently, it is incapable 

 of being directed, because it possesses in itself no 

 active Force enabling it to resist the currents of the 

 air in which it is immersed, and because if it had 

 such a force it would have no fulcrum, or resisting 

 medium against which to exert it. It becomes, as 

 it were, part of the atmosphere, and must go with 

 it where it goes. No bird is ever for an instant of 

 time lighter than the air in which it flies ; but 

 being, on the contrary, always greatly heavier, it 

 keeps possession of a Force capable of supplying 

 momentum, and therefore capable of overcoming 

 any lesser Force, such as the ordinary resistance of 

 the atmosphere, and even of heavy gales of wind. 



