在工程制图中，某些曲面立体相贯时的相贯线的形状非常复杂，以及一些立体模型难以想象。
In engineering drawing the intersectional lines which are formed by spatially intercropping of some curved surfaces have very complicated shape and unthinkable tridimensional model.
这意味着你必须指出,立体图形的底部的曲面，和立体图形的顶部曲面？
So, that means you have to figure out what's the bottom surface of your solid, and what's the top surface of your solid?
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.
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