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airship is for exhibition purposes.
Now let me reason it out in figures.
An envelope pf air surrounds the death from sixty to eighty miles deep, getting lighter and lighter the higher up it is. At the surface of the earth the air pressure is fifteen pounds per square inch, that is, the column of air one inch square, extendinh up for fifty miles, weighs fifteen pounds. Air has a definite weight, as proven by experiment. One thousand cubic feet of air at sea level, weighs seventy two pounds. One thousand cubic feet of hydrogen gas weighs seven pounds, or in other words, the bouyancy of 1,000 cu. feet of hydrogen gas is sixty five pounds, the difference between seventy two and seven. Hydrogen as confined in a balloon may be likened to a bubble under water. The deeper it is the greater bouyancy. Of course, the extreme depth of air is the surface of the earth, which corresponds to the bottom of the sea. 
Now with a working power able to lift sixty five pounds per one thousand cubic feet of balloon, we have a basis on which to figure. For every sixty five pounds that we are to life we must add another one thousand cubic feet to hydrogen gas. So far, so good. We can now figure on the size of the balloon.
But another thing must now be taken into account, the power of the propeller must be made to propel this weight through the air. The resistance of the air is very great and it takes powerful machinery to overcome it. Some of the resistance may be diminished by the shape of