Wagon trains — groups of wagons — rode in four lines across the land when they passed through dangerous country. If attacked, the wagons could quickly form a circle for defense.
马车队(即一群马车)，当他们经过危险区时，他们排成四排向前行进，如果遭受到攻击时，这些马车就会很快地形成一个保护圈进行抵抗。
The idea is to circle the globe in five stages, each taking four or five days of continuous flight (which is about as much as a pilot could endure).
这个环球飞行计划分为五个阶段，每个阶段用时四到五天（这也是一个飞行员所能忍受的极限了）。
In the picture, we can see four circles composing a square, each circle just touches the other two circles.
在画中，我们可以看到四个圆组成了一个正方形，每个圆恰好和其他两个圆接触。
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