The Nature Column.

Digger (Invercargill RSA), Issue 16, 2 July 1920, Page 6

The Nature Column.

("Studeut" wiil be pleased tc receive notes on any branch of Natural History. Observations on birds, insects, plants, etc., will be equally welcome. If using a pen-name, will correspondents please enciose real name and address. ) The constant investigations now being made into the hehaviour of a.ij currents in connection with aeroplanes is tending to throw more light on some of the problems connected with the flight >of birds and insects. A Mr Perry has made some very interesting experiments with wind wheels which have produced some remarkable re» sults. Several forms of van.es. were made, the first with a concave surface and convex back ; a second flat in front with a convex back; a third with both sides quite flat, and a fourth with t-he vane twisted at an angle as in an ordinary windmill, but only to the extent of three degrees. A blast from the electric fan provided the air current and the wheels faced this at right angles. As the air force would meet the vanes of the wheel at right angles it would be expected that no movement of ihe wheel would take place. This was so until the wheels were given a start in either direction, when strange to say they rapidly acquired speeds of over 1000 rovolutions per minute. The flat vane had to be very nicely made and balaneed and staried at a good speed with a piece of string, hut when once staried ran as well as the others. The wheel with vanes given an angle of weather like those of a windmill staried itself and ran as a windmill. When, however, this wheel was started in the reverse direction by hand it continued to revolve in the opposite direction to which it should naturally have taken. Another extraordinary thing discovered was that when the wheels were running the pressure on the surface of the vanes amounted to three times the amouut exerted on them when at rest. It has been demonstrated by Eiffel that a horizontal air currtent meeting an aeroplarm wing without weather angle, i.e., edgeways, the wing being convex above and concave below, a considerable upward thrust is developed. As pointed out above up to three degrees of negative weather angle did not neutralise such thrust. The upward thrust which is only noticeable at considerable speeds, is apparently caused by waves in the air which are crowdecl together in a concave surface, and affect such more strongly than a convex one. Professor Langley who really invented the first practicable airplane wrote a treatise on those waves. It will be seen from the foregoing that rapid horizontal motion through the air of a cnrved surface similar to a bird's wing would produce a considerable amount of uplift.

For many years the soaring flight cf birds has been explained on the aseumption that strong upward currents of air exist, and that soaring birds soixghi these out and were earried upwards, it also being presumed that the wing had a negative angle of weathering. Now it is reasonable to suppose that over hotregions such ascending currents would be found, but this air has to come dowp and we wuold expect to find it so doing about ihe polar regiong. Yet these cold regions are the home of the albatross, one of ihe most notable of soaring birds. I have seen this nohle bird close at hand, and it seems to progress without effort. I also understand from those who have seen in the sub-antarctic islands, that it has considerable difficulty in getting up into the air. The flight of Ihe albatross has always been a puzzle to most people, but

the latest expla, nation of the crowding in of air waves on the under-surface of the wings seems to be a reasonable explanation. It has been held by leading investigators that insects steer themselves in flight by shifting the centre of gravity. Most of us have Roticed the qnickness with which they p-erform their evolutions. Who has not noticed the brilliant dragonfly shooting forward and backward without turning or again turning sharply to either side. The wings of instects beat in miison, and it was thought to be unlikely that they were used for steering. It was considered that an insect bent its abdomen or legs to one side, thus giving it a cant in much the same way as a man on a bicycle leans inward in rounding a corner. This view however is now strongly confcroverted. Long experiments have proved that insects are able to restrict the heating of a wing though the pair still beat in unison. The restriction of the beat causes the oppoeite wing to exert more power and thus turns the insect round. No doubt the sliifting to one side of the centre of gravity is also used to assist in the steering, but this would seem to be subsidiary to pressure steering by the wings and poasibly the t.horax and other paris.