两地间的行程,以直线为最短,这不但为我们人类所公认,就是某些动物也都是有相当的认识的。像猫狗等见了食物,必依一直线奔来:野兔等遇着敌人,必依一直线逃回洞穴,就是显明的例子。直线既然是两地间最短的行程,那末是不是最快速的行程呢?我想有许多人一定会这样说:走最短的路当然可以最快速地到达目的地,那是不成问题的。其实,我们如果用冷静的头脑仔细想上一想,这里面的确存在着问题,决不能肯定说‘最短的行程就是最快的行程’。我们在学习算术的比例的时候,已经知道:‘若 The journey between the two places is the shortest in terms of straight lines. This is not only recognized by us as human beings, but also in certain animals. When cats and dogs meet the food, they must come straight from the line: When the hares meet the enemy, they will flee back to the cave. This is a clear example. Since the straight line is the shortest itinerary between the two places, isn’t it the fastest trip? I think many people will say this: The shortest route can certainly reach the destination most quickly. That is not a problem. In fact, if we think about it with a calm mind, there is indeed a problem in it. We must not say for sure that the shortest journey is the fastest one. When we learned the proportion of arithmetic, we already knew: