insect appears to be available, but its velocity is certainly nearer the latter than the former estimate.
In the case of birds such as those above cited, the soaring mode of flight is so extensively employed that without doubt the process of natural selection, or whatever other method Nature may employ, may be relied upon to have approximated the proportions of least resistance proper to the ordinary velocity of flight of the species. In the case of smaller birds or insects, such as the dragon-fly cited, it is an open question to what extent the problem is modified by the exigencies of active flight, and so the evidence, as confirming or otherwise the present theory, is at the best inconclusive.
§ 188. Considerations Relating to the Form of the Aerofoil.—We have so far specified the form of the aerofoil only so far as the angles and are concerned, and have now not only to discuss the other attributes of the fore and aft section, but also the plan form of the aerofoil and its variation of section from point to point, and in addition the shape it presents when viewed along the axis of flight.
Many of the influences at work to affect the form of the aerofoil do not belong to the province of aerodynamics. The question of form, as viewed along the axis of flight, is governed almost entirely by aerodonetic considerations, and the discussion of this point will therefore be reserved.
The present subject has already been examined in Chap. IV., § 120 ; it remains for us now to continue the discussion in the light of the present theory.
If we suppose, provisionally, that the aerofoil section is of the form of the arc of a circle, then such a form would manifestly carry out the requirements of hypothesis with a uniform distribution of pressure on its surface, for we are supposing that the “fluid” consists of a limited layer composed of a number of strata whose individual continuity is preserved, after the manner of Fig. 108. If we suppose such an aerofoil to be gliding in a
