2 Rayleic.ii, The Mechanical Principles of Flight. 



observations, are familiar with mechanical principles, and 

 thus statements are often put forward which amount to 

 mechanical impossibilities. The arm-chair theorist at 

 hoinc, on the otlu'r liaiul, may be too willinir to discredit 

 reports of actual observations, especially when the)- are 

 made in other parts of the world. On both sides it seems 

 to be admitted that there is no sailing flight in the absence 

 of wind ; but observers, untrained in dynamics and mis- 

 led by the analogy of the kite, are apt to suppose that 

 the existence of wind at once removes the difficulty. The 

 doctrine of relative motion shows however that, so long as 

 there is no connection with the groimd, a uniform hori- 

 zontal wind is for this purpose the same thing as absolutely 

 still air. 



In a short paper upon this subject {Nafnre, vol. xxvii., 

 p. 534, 1883) I pointed out that, "Whenever a bird 

 pursues his course for some time without working his 

 wings, we must conclude either (i) that the course is not 

 horizontal, (2) that the wind is not horizontal, or (3) that 

 the wind is not uniform. It is [)robable that the truth is 

 usually represented by (i) or (2), but the question I wish 

 to raise is whether the cause suggested by {3) may not 

 sometimes come into operation." Case(i) is that of a 

 rook gliding downwards from a tree in still air with 

 motionless wings. We shall presently consider- upon what 

 conditions depend the time and distance of travel possible 

 with a given descent. Case (2) is clo.sely related to 

 case (i). If the air have an upward velocity equal to that 

 at which the rook falls through it in a vertical direction, 

 the vertical motion is compensated, and the course of the 

 rook relatively to the ground becomes horizontal. It is not 

 neces.sary, of course, that the wliole motion of the air be 

 upwards ; a horizontal motion of the air is simply super- 

 posed. A bird gliding into a wind having a small upward 



