Page:Popular Science Monthly Volume 28.djvu/17

 Of course we may provide our balloon with wings or propeller, and fly as the birds fly. This has been and continues to be a favorite combination with our inventors. One patented in this country in 1880 has been chosen as an illustration. The balloon, oblong in shape and divided for safety into compartments, supports a car containing the propelling machinery, and also a gas-generator to make up such loss of hydrogen as may occur. Two immense rudders steer the machine. It is propelled by four paddle-wheels, which would act, one would think, very much as the wheels of our river-steamers would act, if totally immersed in the water, and would be about as likely to drive the balloon backward as forward.

Generally, however, in machines of this class the propeller is one gigantic screw, or a number of screws, and the balloons have a variety in shape and grouping which is quite remarkable.

It is strange that people have not realized that a thing necessarily so big and light as a balloon can not be made strong and durable enough to stand the pressure of the wind at comparatively low velocities. Floating with the current, the velocity would have no destructive effect; but brought into opposition to this current, or forced at any great speed through the air, the resistance would be much greater than a silk bag could safely stand.

It may be well here to refer to a table giving the relation of pressure to velocity of air, experimentally determined and verified time and again—results very important in the study of flying and flying-machines:

Now let us suppose that a balloon only forty feet in diameter should resist the pressure of wind blowing at the rate of twenty miles an hour, or, what is the same thing, that the balloon should be traveling through still air at this speed. The surface presented to the wind would be about twelve hundred square feet, and the pressure on each square foot, from our table, would be 1·9 pound, and the total pressure over a ton. A calculation is hardly necessary to show that such a pressure,