Sometimes the craft is heavier than air, and sometimes lighter.

有时飞行器比空气重, 有时轻。

On December seventeen, nineteen-oh-three, they made the world's first flight in a machine that was heavier than air and powered by an engine.

1903年12月17日，他们制造了世界上第一架飞机，以引擎为动力。

We've installed tubes throughout the theater, lower to the ground, because CO2 is heavier than air.

我们在整个剧院都安装了管道，靠近地面， 因为二氧化碳比空气重。