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Strictly speaking, air is mostly nitrogen–about seventy-eightpercent, and about twenty-one percent oxygen, which leavesabout one percent of other stuff, like water vapor and carbondioxide.
严格地说，空气主要由大约78%的氮气和21%的氧气组成，另外还有1%的其他气体，像水蒸气和二氧化碳。
So air is mostly empty space?
所以，空气大部分都是真空吗？
Yep, in fact, if you ever condensed a gallon of air into a solid,you'd end up with a little over one tenth of one percent of the air's original volume.
是的。实际上，如果你将一加仑的空气凝结成固体，这个固体的体积只比空气原本体积的千分之一多一点。
And the reasonwe don't notice all this emptiness is because air molecules are constantly moving around at thespeed of sound, and bouncing against any surface that happens to fall in their path.
我们没有注意到这一点的原因是空气分子以音速在不停地移动，而当它们碰到任何表面时都会反弹回去。
So we're constantly bombarded by air molecules.
那么，我们一直被空气分子撞击着，
So how come we don't feel that?
可是为什么我们感觉不到呢？
Well, actually we do.
其实我们能感觉到的。
The force these air molecules exert is what we measure when we measureair pressure.
空气分子施加的压力就是我们平时测量的气压。
