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PROGRESSION IN OR THROUGH THE AIR. 135 



of body, twelve inches ; expanse of wing from tip to tip across 

 the body, five feet nine inches ; widest portion of wing across 

 primary and tertiary feathers, eleven inches ; across secondary 

 feathers, twelve inches. 



Each wing, when carefully measxired and squared, gave an 

 area of twenty-six square inches. The wings of the heron, 

 consequently, furnish a supporting area of four feet four inches 

 square. As the bird only weighs 3 lbs. 3 ounces, this gives 

 something like twenty-six square inches of wing for every 

 25 1 ounces of bird, or one foot 5 J inches square for every 

 1 lb. 1 ounce of body. 



In the gannet there is only one foot one square inch of 

 wing for every 2 lbs. 4^ ounces of body. The gannet has, 

 consequently, less than half of the wing area of the heron. 

 The gannet's wings are, however, long narrow wings (those 

 of the heron are broad), which extend transversely across the 

 body; and these are found to be the most powerful — the 

 wings of the albatross — which measure fourteen feet from tip 

 to tip (and only one foot across), elevating 18 lbs. without 

 difficulty. If the wings of the gannet, which have a super- 

 ficial area of three feet three inches square, are capable of 

 elevating 7 lbs., while the wings of the heron, which have a 

 superficial area of four feet four inches, can only elevate 3 lbs., 

 it is evident (seejng the wings of both are twisted levers, and 

 formed upon a common type) that the gannet's wings must 

 be vibrated with greater energy than the heron's wings ; and 

 this is actually the case. The heron's wings, as I have ascer- 

 tained from observation, make 60 down and 60 up strokes 

 every minute ; whereas the wings of the gannet, when the 

 bird is flying in a straight line to or from its fishing-ground, 

 make close upon 150 up and 150 down strokes during the 

 same period. The wings of the divers, and other short- winged, 

 heavy-bodied birds, are urged at a much higher speed, so that 

 comparatively small wings can be made to elevate a compa- 

 ratively heavy body, if the speed only be increased suffi- 

 ciently.^ Flight, therefore, as already indicated, is a ques- 



1 The grebes among birds, and the beetles among insects, furnish examples 

 where small wings, made to vibrate at high speeds, are capable of elevating 

 great weights. 



