(1) “Animal Flight, ” p. 299.

(2) “Le vol sans battement,” iby L.. P, Mouillard. (Paris, 1912, Librarie Aeronautique.)

(3) In “Animal Flight,” p. 204, I have based some theoretical considerations on the steadiness of the wing–tips in flex–gliding flight. Mouillard's results may be quoted in criticism of these considerations. My description includes observations of the position or changes of position of individual gull feathers, and it seems strange that I could have overlooked vibration of their tips if it occurred in the cases that came under my notice. It is fair to point out that Mouillard's observations did not go very far. For instance, he mistook the “double dip ” movement for a signal to other birds. He would scarcely have arrived at this conclusion had he noticed the increase of speed that follows it, or the rotation downwards of the wings that takes place during the up–stroke of the movement. Lest it should be thought that soaring flight is due to previously undiscovered wing movements revealed by these observations, I may refer to “Animal Flight,” p. 259, and to my paper “On the Flight of Albatrosses and Gannets” (Aeronautical Journal, Vol. XIX., pp. 22-9), where very clear reasons are adduced for regarding this suggestion as inadmissible.

(4) “Animal Flight,” p. 299.

(5) “On the Flight of Albatrosses and Gannets,” Aeronautical Journal, Vol.XIX., pp. 22–9Google Scholar

(6) “Animal Flight, ” pp. 169, 258 and 391.

(7) “Animal Flight,” pp. 20 and 283.

(8) “Animal Flight,” pp. 372, 373 and 377. Two other observers besides myself have seen the hill–crow soaring in sunshine in the absence of appreciable wind. One of these observers asserts that the hill–crow does not soar in the presence of wind. The other observer informs me that he has seen it soaring in wind under heavy cloud, but I am not satisfied that this was not merely a case of the bird taking advantage of ascending currents.

(9) “Animal Flight,” p. 180.

(10) ”Animal Flight,” pp. 365 and 383.

(11) Aeronautical Journal, July, 1914, Vol. XVIII., p. 191.Google Scholar

(12) See “Animal Flight,” pp. 34 and 371.

* The following account of this observation is from my diary:—

“29th November, 1913. In Red Sea travelling north. 10.30 a.m. Two steamers seen on horizon travelling in opposite directions. The smoke of each steamer formed a zigzag trail as occurs in the presence of wind. The surface of the sea was glassy as far as the eye could see. That there was no wind at a height in the direction in which the steamers were moving is proved by the fact that the two trails of smoke remained at the same height above the water. That there was no wind at a height in a direction making an angle with the course of the steamers is made probable by the fact that the smoke of my steamer was now passing directly astern. In the case of each steamer the smoke at the lower curves of the zigzag became diffuse and dissolved more rapidly than did the smoke at the summit of the curves.

“ 10.36 a.m. One small wind patch in sight. The steamer took 22 seconds to pass through it travelling along its longest diameter. The speed of the steamer was about 8 metres per second. Therefore the length of this cat's paw. was about 176 metres.”

(13) It would be of interest to see whether any other cases can be quoted of wind showing a decrease of soarability as it acquires an upward trend. For instance, in the formation of a small local thunderstorm there is “a rising inflow of warm surface winds” (see Aeronautical Journal, Vol. XVIII., pp. 351 and 352). This description given by Davis corresponds to what I have described as the “attraction wind” of a dust storm, in which a decrease of soarability is always observed (see “Animal Flight,” p. 302). But I know of no adequate proof that the air in which I have observed this decrease of soarability was actually moving with an upward trend.

(14) It must not be forgotten that while some winds lead to the appearance of a soarable area in which gulls remain in continued gliding flight, other winds lead to the appearance of a soarable area in which the gliding is interrupted by more or less flapping. In yet other winds gulls arriving at the position of the soarable area find it necessary to flap with full strength in order to keep up with the ship. A change from one condition to another of the soarable area may be seen on the same day on the same steamer. No facts as yet observed show any relation between the strength or direction of the wind and the presence or absence of a soarable area (see “Animal Flight,” p. 254). The development of this area seems to depend on the presence of some as yet unrecognised character in the wind. Comparison may be made with the soarable and unsoarable winds that I have described in “Animal Flight,” pp. 278 et seq.

(15) Among similar instances may be mentioned the extraordinary and unexplained increase of speed that follows the “double–dip movement” in fast flex–guiding. This appears to “initiate an increased rate of use of air energy” (“Animal Flight,” p. 201). Another instance is the bent–up position of the digital quill feathers in fast flex–gliding flight (loc. cit., p. 202). The meaning of this bending up is as follows: I have elsewhere adduced reasons for believing that some of the more efficient of the soaring birds have a bad gliding angle, and that the shape of their body and wings is such that considerable force is required to drive them ahead at speed through the air (“Animal Flight,” Chapter XIX). The amount of this force is indicated by the position to which the wingtip feathers are bent up in fast flex–gliding flight (see “Animal Flight,” p. 202, Fig. 69). If the wing of a vulture is held in one hand it requires all the force that can conveniently be exerted by the other hand to bend up the wing–tip feathers to the position they assume (under air pressure) in fast flex–gliding flight. The pressure on the wing–tip feathers has to be exerted in an upward and forward direction to bring them to the correct position, thus indicating the direction in which the unknown force of soarability is operative.