riot, with an admirable audacity and industry, built and smashed monoplane after monoplane until he had evolved the highly successful "Blériot VIII.," the first monoplane in the world to make extended trips.

The astonishing progress in aviation was on, and as it rolls and grows in size like the proverbial ball of snow, we should pause and reflect upon the immense value of the work of Langley, Lilienthal, and Chanute.

CHAPTER II.

THE RESISTANCE OF THE AIR AND THE PRESSURE ON NORMAL PLANES

ALTHOUGH the fact that air has inertia is a familiar one, the important deductions to be drawn therefrom, were not fully recognized until the classic experiments of Langley exhibited them in their true import.

The resistance of the air in its bearing upon aeronautics, and especially in the consideration of the pressure on the surface of an aeroplane, is of fundamental importance.

Many values and methods of determining air resistance have been suggested, but they differ widely from each other. Because of this, designers of aeroplanes experience great difficulty in calculating the probable performance of their machines. A small difference in the value of the "constant of air resistance" may mean an over or under estimation of a certain pressure to the extent of several pounds, which in turn may involve added expense and decreased efficiency.

It is therefore desirable to investigate the present knowledge on the subject, not so much for the purpose of theoretic discussion as to arrive at some definite and conclusive values of the various quantities involved, that will be of use to the engineer.

The resistance of the air is directly proportional to its density. The density of the air varies with (1) temperature, (2) pressure, and (3) its state of equilibrium.

An increase of temperature causes air to expand, and therefore the density diminishes. Roughly, the density of the air varies inversely by 0.36 per cent for a difference of 1 deg. C.

At sea level in our latitudes and at 0 deg. C. 1 cubic foot of air

weighs very nearly 11⁄2 ounces if the pressure is at 760 millimeters of mercury. But this pressure decreases as the height above sea level increases. and also at any point is subject to great variations due to meteorological conditions. A difference in pressure of 7.6 millimeters causes a direct variation of the density of about 1 per cent. At 20 deg. C. a difference in height of 340 feet above sea level gives a difference of 10 millimeters in the pressure. At a height of about 18,000 feet, for instance, the density of the air is exactly one-half of that at sea level.

It is only recently that the effect of the condition of equilibrium

AN INSTANTANEOUS PHOTOGRAPH BY PROF.
MAREY, SHOWING THE ACTION OF AN
AIR STREAM PASSING A NORMAL
SURFACE FROM LEFT TO RIGHT

Note the whirls and regions of discontinuity
and the compression of the air stream in
front of the surface. These marvelous photo-
graphs were obtained by admitting thin
streams of smoke into the air current.

of the air at any one point upon the density has been considered. The temperature and the pressure in a certain region remaining constant. a gusty wind and several buildings, etc., being in the neighborhood, there would be large variations in the density at different points. The disturbances and eddies set up by normal planes, spheres and spindles, are clearly shown in the accompanying stream line photographs. Even an aeroplane with an arched surface will, if the speed is high enough, leave a region of high density below and in its wake, and a region of low density above and in its wake. Everywhere in the atmosphere, and especially on windy days, there exist "pockets" of high density and of low density, sometimes large enough to completely immerse a full-sized aero

plane. Very often the nature of a country is such that, when the wind comes from a certain direction, a region of low density always forms at some particular point. Abroad at the Rheims aerodrome, and here at our flying grounds at Mineola, such points actually exist, always about in the same place, and are called by the aviators "air holes." An aeroplane entering one of these lowdensity regions from the air of higher density around it, will suddenly fall without any warning, merely because the pressure has

enormously decreased, and the aeroplane has not had time to attain the requisite velocity of support in this lighter medium. Then again, when the machine after this experience passes into the heavier surrounding air, the shock due to the suddenly increased pressure is likely to cause a straining of some part, and a possible breakage. Whenever considering the air in which an aeroplane is flying, we must never lose sight of the fact that this fluid is irregular and unstable in its flow, subject to the most intricate movements and treacherous to the last degree.

The density, therefore, varies greatly, and directly affects the pressures on an aeroplane. In the summer, on a dry clear day, the