The Man Angel was lighter-than-air and had paddles like a rowboat that the aviator could pump.
“人类天使”比空气还轻，有像划艇一样的桨，飞行员可以摇动它们。
2
Helium is far lighterthanair and is inert, which means it won't combust when you combine it with air and energy, like Hydrogen does (below).
氦气比空气轻得多并且是惰性气体，这意味着，当它与空气和明火接触不会燃烧，而氢气却会(请看下图)。
3
Hydrogen is lighterthanair, but compressing it, storing it in tanks and adding all the necessary equipment starts to tip the scales towards heavy again.