Staying aloft when hovering and flying slowly is demanding. According to quasi-steady-state aerodynamic theory, slow-flying vertebrates should not be able to generate enough lift to remain aloft. Therefore, unsteady aerodynamic mechanisms to enhance lift production have been proposed. Using digital particle image velocimetry, we showed that a small nectar-feeding bat is able to increase lift by as much as 40% using attached leading-edge vortices (LEVs) during slow forward flight, resulting in a maximum lift coefficient of 4.8. The airflow passing over the LEV reattaches behind the LEV smoothly to the wing, despite the exceptionally large local angles of attack and wing camber. Our results show that the use of unsteady aerodynamic mechanisms in flapping flight is not limited to insects but is also used by larger and heavier animals.

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There are two outstanding problems relating to the propagation of wireless waves over the earth’s surface which at present remain unsolved, viz.— What is the agency which causes the waves to follow the curvature of the earth, thus rendering long-distance communication possible ? And what is the cause of the large and rapid variations of the intensity and apparent direction of the waves, very commonly observed at the receiving station and confined almost entirely to the hours of darkness ? Both phenomena can be explained to some extent by the well-known Heaviide-layer theory, with the modifications proposed by Eccles, but it is generally admitted that further experimental evidence of the existence of the layer is needed. If the theory is correct and is sufficient, it follows that, a receiver experiencing either of the above phenomena, part of the energy must be arriving in a downward direction ( i. e ., inclined to the horizontal); and that during the occurrence of directional variations, this downcoming wave must have at horizontally polarised component ( i. e ., with its electric force horizontal). I has, therefore, been generally recognised for some time that a conclusive experimental demonstration of the presence or absence of such waves, by suit able quantitative measurements in three dimensions, would aid considerably in proving or disproving the Heaviside-layer theory.

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