这个流量度量了，单位时间内通过s的烟雾的多少。
So, this flux measures how much smoke is passing through s per unit time.
单位时间内的流量就是一个小立体，它的底是曲面上的一块，它的高由向量场给出。
Well, it is the stuff that lives in a small box whose base is that piece of surface and whose other side is given by the vector field.
看这儿，左边的这个是单位时间内,从区域里出来的流量。
Physically, see, this guy on the left is the total amount of stuff that goes out of the region per unit time.
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