CONTRIVANCE A NECESSITY. 137 



blem to be solved in flight, that the engine which 

 works the wings should be very strong, very com- 

 pact, of a special form, and that, though heavier 

 than the air, it should not have an excessive 

 weight. These conditions are all met in the power, 

 in the outline, and in the bulk of the pectoral 

 muscles which move the wings of birds. Few per- 

 sons have any idea of the force expended in the 

 action of ordinary flight. The pulsations of the 

 wing in most birds are so rapid that they cannot be 

 counted. Even the Heron seldom flaps its wings 

 at a rate of less than from 120 to 150 strokes in 

 a minute. This is counting only the downward 

 strokes, preparatory to each one of which there 

 must be an upward stroke also : so that there are 

 from 240 to 300 separate movements per minute. 

 Yet the Heron is remarkable for its slow and heavy 

 flight, and it is difficult to believe, until one has 

 timed the pulsations with a watch, that they have 

 a rapidity approaching to two in a second. But this 

 difficulty is an index to the enormous comparative 

 rapidity of the faster-flying birds. Let any one 

 try to count the pulsations of the wing in ordinary 

 flight of a Pigeon, or of a Blackcock, or of a Part- 



