在一些工业地区，马匹正沿着金属轨道拉着满载货物、带法兰轮子的马车；固定的蒸汽机在工厂和矿井里噗噗作响。
In some industrial regions, heavily laden wagons, with flanged wheels, were being hauled by horses along metal rails; and the stationary steam engine was puffing in the factory and mine.
安全带法律是怎样影响汽车安全的？
为了找到带法向量的面元，我就取这两个的叉积。
To find the surface element together with a normal vector, I would just take the cross-product between these guys.
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