Diagram showing disturbance of wind currents by inequalities of the ground, and the smoother currents of the upper air. Note the increase of density at A and B, caused by compression against the upper strata.
Three properties of the air enter into the sum total of its resistance—inertia, elasticity, and viscosity. Inertia is its tendency to remain in the condition in which it may be: at rest, if it is still; in motion, if it is moving. Some force must be applied to disturb this inertia, and in consequence when the inertia is overcome a certain amount of force is used up in the operation. Elasticity is that property by virtue of which air tends to reoccupy its normal amount of space after disturbance. An illustration of this tendency is the springing back of the handle of a bicycle pump if the valve at the bottom is not open, and the air in the pump is simply compressed, not forced into the tire. Viscosity may be described as “stickiness”—the tendency of the particles of air to cling together, to resist separation. To illustrate: molasses, particularly in cold weather, has greater viscosity than water; varnish has greater viscosity than turpentine. Air exhibits some viscosity, though vastly less than that of cold molasses. However, though relatively slight, this viscosity has a part in the resistance which opposes the rapid flight of the airship and aeroplane; and the higher the speed, the greater the retarding effect of viscosity.
The inertia of the air, while in some degree it blocks the progress of his machine, is a benefit to the aeronaut, for it is inertia which gives the blades of his propeller “hold” upon the air. The elasticity of the air, compressed under the curved surfaces of the aeroplane, is believed to be helpful in maintaining the lift. The effect of viscosity may be greatly reduced by using surfaces finished with polished varnish—just as greasing a knife will permit it to be passed with less friction through thick molasses.
In the case of winds, the inertia of the moving mass becomes what is commonly termed “wind pressure” against any object not moving with it at an equal speed. The following table gives the measurements of wind pressure, as recorded at the station on the Eiffel Tower, for differing velocities of wind:
| Velocity in Miles per Hour | Velocity in Feet per Second | Pressure in Pounds on a Square Foot |
|---|---|---|
| 2 | 2.9 | 0.012 |
| 4 | 5.9 | 0.048 |
| 6 | 8.8 | 0.108 |
| 8 | 11.7 | 0.192 |
| 10 | 14.7 | 0.300 |
| 15 | 22.0 | 0.675 |
| 20 | 29.4 | 1.200 |
| 25 | 36.7 | 1.875 |
| 30 | 44.0 | 2.700 |
| 35 | 51.3 | 3.675 |
| 40 | 58.7 | 4.800 |
| 45 | 66.0 | 6.075 |
| 50 | 73.4 | 7.500 |
| 60 | 88.0 | 10.800 |
| 70 | 102.7 | 14.700 |
| 80 | 117.2 | 19.200 |
| 90 | 132.0 | 24.300 |
| 100 | 146.7 | 30.000 |
In applying this table, the velocity to be considered is the net velocity of the movements of the airship and of the wind. If the ship is moving 20 miles an hour against a head wind blowing 20 miles an hour, the net velocity of the wind will be 40 miles an hour, and the pressure 4.8 lbs. a square foot of surface presented. Therefore the airship will be standing still, so far as objects on the ground are concerned. If the ship is sailing 20 miles an hour with the wind, which is blowing 20 miles an hour, the pressure per square foot will be only 1.2 lbs.; while as regards objects on the ground, the ship will be travelling 40 miles an hour.
Apparatus for the study of the action of air in motion; a blower at the farther end of the great tube sends a “wind” of any desired velocity through it. Planes and propellers of various forms are thus tested.
Systematic study of the movements of the air currents has not been widespread, and has not progressed much beyond the gathering of statistics which may serve as useful data in testing existing theories or formulating new ones.
It is already recognized that there are certain “tides” in the atmosphere, recurring twice daily in six-hour periods, as in the case of the ocean tides, and perhaps from the same causes. Other currents are produced by the earth’s rotation. Then there are the five-day oscillations noted by Eliot in India, and daily movements, more or less regular, due to the sun’s heat by day and the lack of it by night. The complexity of these motions makes scientific research extremely difficult.