BIRDS CLIMBING THE AIR. 213 



impossible to determine; but to me it does not appear 

 as if the hawk did it from necessity. It has more 

 the appearance of variation : just as you or I might 

 walk fast at one moment and slowly at another, now 

 this side of the street and now the other. A shifting 

 of the plane of the wings would, however, in all prob- 

 ability, give some impetus : the question is, would it 

 b^ sufficient ? I have seen hawks go up in sunn}' 

 and lovely weather in fact, they seem to prefer still, 

 calm weather; but> considering the height to which 

 they attain, no one can positively assert that they do 

 or do not utilize a current. If they do, they may be 

 said to sail (a hawk's wings are technically his sails) 

 round half the circle with the wind fair and behind, 

 and then meet it the other half of the turn, using the 

 impetus they have gained to surmount the breeze as 

 they breast it Granting this mechanical assistance, 

 it still remains a wonderful feat, since the nicest ad- 

 justment must be necessary to get the impetus sufficient 

 to carry the birds over the resistance. They do not 

 drift, or very little. 



My own impression is that a hawk can soar in a 

 perfectly still atmosphere. If there is a wind he uses 

 it ; but it is quite as much an impediment as an aid. 

 If there is no wind he goes up with the greater ease 

 and to the greater height, and will of choice soar in a 

 calm. The spectacle of a weight for of course the 

 hawk has an appreciable weight apparently lifting 

 itself in the face of gravitation and overcoming friction, 

 is a very striking one. When an autumn leaf parts on 

 a still day from the twig, it often rotates and travels 

 some distance from the tree, falling reluctantly and 



