are constructed in such a manner as to be capable of great 

 expansion when struck in a downward direction. K we except, 

 in this action, the slight hollow which takes place on the 

 xmder-side, they become almost two planes. In order that 

 the downward action may be accomplished to the necessary 

 extent, the muscles which mo¥e the wings have been made 

 exceedingly large; so large, indeed, that, in some instances, 

 they have been estimated at not less than a sixth of the weight 

 of the whole body. Therefore, when a bird is on the grotmd 

 and intends to fly, it takes a leap, and immediately stretching 

 its wings, strikes them out with great ferce. By this act these 

 are brought into an oblique direction, being turned partly 

 upwards and partly horizontally forwards. That part of the 

 force which has the upward tendency is neutralized by the 

 weight of the bird, whilst the horizontal force serves to carry- 

 it forward. The stroke being completed, it moves upon its 

 wings, which, being contracted and having their edges turned, 

 upwards, obviate, in a great measure, the resistance of the air. 

 When it is sufficiently elevated, it makes a second stroke 

 downwards, and the impulse of the air again moves it for- 

 ward. These successive strokes may be regarded as so many 

 leaps taken in the air. When the bird desires to direct its 

 course to the right or the left, it strikes strongly with the 

 opposite wing, which impels it to the proper side. In the 

 motions of the animal, too,, the tail takes a prominent part, 

 and acts like the rudder of a ship, except that, instead of 

 sideways, it moves upwards and downwards. If the bird 

 wishes to rise, it raises its tail ; and if to fall, it depresses it ; 

 and, whilst in a horizontal position, it keeps it steady. There 

 are few who have not observed a pigeon or a crow preserve, for 

 some time, a hor&ontal flight without any apparent motion of 

 the -vrings. This is aceqmpHshed by the bird having already 

 acquired sufficient valooity, and its -v^gs being parallel to the 

 horizon, meeting, with but small resistance froin the atmo- 

 sphere. If it begins to fall, it can easUy steer itself upward 

 by means of its tail, tiU the motion it had acquired is nearly 

 spent, when it must be renewed by a few more strokes of the 

 wings. On alighting, a bird expands its wings and tail foUy 

 against the air, as a ship, in tacking round, backs her sails, in 

 order that they may meet with all the resistance possible. 



" The anterior extremities of birds," says MaogiUivray, " are 

 modified so as to render them subservient to aerial progression. 

 They are converted into wings by having appended to them a 



