How are horsepower and standard displacement for a rigid airship calculated according to this book?
The book author just eliminated Hp from 1 by the formula 2.
$$ 15000 + 44(.39D^{2/3}) = .357 D$$
Do arthitmatic and rearrange:
$$.357 D -17.16 D^{2/3} = 42000$$
Divide by the .357 to get
$$ D- 48.067 D^{2/3} = 42016.8 $$
Then round off because he used a slide rule not a calculator.