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HARDWICKE'S SCIENCE-GOSSIP. 



THE WINGS OF BIRDS. 

 By J. Lancaster. 



[Continued from p. 224.] 



LET us now attend to a matter of simple experi- 

 ment, which demonstrates bird flight, as I have 

 presented it in the "American Naturalist," and which 

 connects the above described construction of a wing, 

 which is the organ of flight, with the mechanical 

 forces which operate upon it. A few cents' worth 

 of material and the expenditure of a little time is 

 required, which, coupled with a minimum of me- 

 chanical expertness, will suffice. The movement 

 following changes made in the plane are so prompt 

 and emphatic that there is no mistaking them. 



If we take a sheet of light strong paper, such as goes 

 into construction of fireworks, and stretch sixteen by 

 forty inches of it tightly on thin strips of bamboo, so 

 as to present a plane surface on one side, and fasten a 

 four-ounce weight in the centre beneath by threads to 

 the four corners, we shall have a miniature parachute. 

 By putting its upper surface against the ceiling, and 

 permitting it to fall, it will descend vertically, if we 

 have it well balanced. It is obvious that the entire 

 column of air, from the ceiling to the floor, is moved 

 out of the place it occupied by the fall of the plane 

 upon it. It is evident that the pressure of the plane 

 against the air is greatest in the centre, diminishing 

 to the edges where it is nothing. Hence the zone of 

 air under the plane acted upon by it, would be 

 pyramid-shaped with a base sixteen by forty inches, 

 a top line twenty-four inches long and a height 

 varying with the velocity of the fall of the plane, with 

 the apex pointing downwards, and there would be a 

 steady flow or slipping of the compressed air along the 

 surface in all directions from the centre. This slipping 

 would cause friction on the surface, which being alike 

 on all directions, no lateral motion would occur. If 

 we now take some large-grained, light sawdust and 

 mucilage, and thinly sprinkle one-half of the under- 

 surface from one of the long edges to the centre, the 

 level plane will no longer fall vertically, but more 

 towards the edge bounding the sawdusted half. If 

 we extend the paper an inch over the edge opposite 

 the sawdust and sink it into the bamboo, so that it 

 will curve upwards, we shall find on dropping the 

 plane that its motion sideways will be augumentcd. 

 If we paste a slip of paper to the front edge, allowing 

 it to project one inch and a half beneath, we shall 

 find the motion sideways to be further increased so as 

 to be many times that of the fall. If we take the plane 

 to the house top, when the air is calm, and allow it to 

 drop, it will describe a curve while it is being accele- 

 rated, but on uniform motion occurring it will slant 

 downwards very gently and move laterally from six to 

 eight times as fast as it falls vertically. This experi- 

 ment is very delicate, and the motion varies greatly, 

 depending on the weight and humidity of the air and 

 the construction of the plane. The cause of this 



behaviour is obvious. The air is rushing along the 

 under side of the plane to escape, drags the rough 

 surface with it to the front, slipping over the smooth 

 •part to the rear easily. Then as it turns the curve it 

 expands against it, augmenting the front thrust. 

 Where the air escaping towards the front meets the 

 ledge, a complete stoppage occurs, and the thrust 

 against it is quite violent. The ledge in front of the 

 roughened half of the surface, and the rear curve, all 

 tend to throw the plane edgeways in the same 

 direction while it falls — the ledge by directly 

 stopping the motion of the rushing air, the rough 

 surface by frictional resistance, and the curve by pre- 

 senting a base for the compressed air to expand 

 against. The force required to produce these various 

 results is put into the air by the descent of the plane 

 upon it under the impulse of the gravity of its mass. 

 The wing of a bird would act in the same way that 

 the plane acts, and for the same reason. The elastic 

 feather tips give the curve. The construction of the 

 feather surfaces gives the smooth rear, and rough 

 front part ; the feather surfaces give the smooth rear, 

 and rough front part ; and the front projection of 

 the integument covering the bones forms the ledge. 



Why is it that the plane moves so much faster 

 edgeways when it falls ? Obviously because the con- 

 densed air in contact with the surface moves with far 

 higher velocity than the fall, canying the plane with 

 it. The inertia of the air resists sudden motion, so 

 that the particles are compressed before they begin to 

 move, and when motion at length occurs, it is 

 sufficiently rapid to make up for lost time. A non- 

 elastic fluid would not present this rapid motion on 

 the compressing surfaces. If we increase the pendent 

 weight and take the plane to the lantern of a light- 

 house, or very high building, we shall find that its 

 lateral motion will be greater, and every addition of 

 weight will heighten the lateral velocity in a greater 

 ratio than the fall. The reason is that the plane 

 meets with small resistance to its motion edgeways, 

 so that a slight increase of force in this direction 

 moves it rapidly. But, however great this motion 

 may be, however closely it may approach level 

 translation, by no means could it become horizontal 

 as long as the plane itself was level. The entire force 

 producing lateral motion comes from motion in the 

 direction of gravity down the vertical, and horizontal 

 translation would stop the fall, thus cutting off the 

 motive power. But if we slant the plane the vertical 

 goes over with it. Its own activity determines the 

 direction of its vertical, it is fluid pressures we are 

 dealing with, and what would be level translation on 

 our horizontal is a constant fall from the bird's 

 horizontal ; and a soaring bird certainly needs no 

 further explanation, neither does a wild duck. It is 

 not heavy enough, relatively to the size of its wings, 

 to reach a uniform velocity of fall which would pro- 

 duce the thrust needed in the lateral motion, and 

 resort to flapping is compelled to aid the normal 



