“As soon as a lighter fluid lies above a denser one with well-defined boundary, then evidently the conditions exist at this boundary for the origin and regular propagation of waves, such as we are familiar with on the surface of water. This case of waves, as ordinarily observed on the boundary surfaces between water and air, is only to be distinguished from the system of waves that may exist between different strata of air, in that in the former the difference of density of the two fluids is much greater than in the latter case. It appeared to me of interest to investigate what other differences result from this in the phenomena of air waves and water waves.
“It appears to me not doubtful that such systems of waves occur with remarkable frequency at the bounding surfaces of strata of air of different densities, even although in most cases they remain invisible to us. Evidently we see them only when the lower stratum is so nearly saturated with aqueous vapor that the summit of the wave, within which the pressure is less, begins to form a haze. Then there appear streaky, parallel trains of clouds of very different breadths, occasionally stretching over the broad surface of the sky in regular patterns. Moreover, it seems to me probable that this, which we thus observe under special conditions that have rather the character of exceptional cases, is present in innumerable other cases when we do not see it.
“The calculations performed by me show, further, that for the observed velocities of the wind there may be formed in the atmosphere not only small waves, but also those whose wave lengths are many kilometers which, when they approach the earth’s surface to within an altitude of one or several kilometers, set the lower strata of air into violent motion and must bring about the so-called gusty weather. The peculiarity of such weather (as I look at it) consists in this, that gusts of wind often accompanied by rain are repeated at the same place, many times a day, at nearly equal intervals and nearly uniform order of succession.”
Commandant Le Clement de Saint-Marcq has drawn some interesting conclusions from the hypothesis that an ordinary wind consists of a uniform current on which is superposed periodic motions in the wind’s main direction and also at right angles thereto. But he has not established his hypothesis by adequate observations. He assumes the pulsations to be simple harmonic motions, which of course they would be if they were plane compressional waves; but at the same time he shows that the fluctuations are too large to be compressional waves, with the concurrent slight variations of the barometric pressure.
It is still a question whether the pulsations of the natural wind be harmonic. If so, the speed records should be sine curves, and the to and fro acceleration of any mass of moving air should be variable for any given pulsation. But the few records available show in many parts a constant acceleration of the wind speed throughout a particular swell or lull of velocity, indicating that the pulsations are not generally simple harmonic ones.
In scanning the wind-speed records published by Langley, so many instances of uniform wind acceleration are noticed that one naturally inquires whether the rate of gain of velocity be sufficient to sustain in soaring flight an aëroplane or bird held to the wind solely by its inertia, as Langley believed to be possible. The total forward resistance of a well-formed aërial glider, or bird, may be taken as one eighth of its weight; hence, if poised stationary in its normal attitude of flight, it will just be sustained by a direct head wind having a horizontal acceleration of one eighth that of gravity, or four feet per second. Now, the most favorable parts of the record here shown ([Fig. 58]) exhibit nowhere an acceleration so great as four feet per second, and on the average far less than that, as may be proved by sealing the diagram. Hence, the wind here recorded was wholly inadequate to support by its pulsative force either bird or man. But as this record is a fair representative of all those published by Dr. Langley, it follows that such pulsations can at best merely aid in soaring when happily and adroitly encountered; but that they cannot fully sustain soaring at any level, much less during ascensional flight to great altitudes, or migrational flight to vast distances. It still remains, therefore, to ascertain what kind of aërial currents are adequate to sustain those marvelous feats of soaring on passive pinions which for ages have been the delight and wonder of all keen observers, and which are of such enduring interest to mankind. This investigation, however, appertains more particularly to the science of applied aërodynamics.