海崖脚下的淡水

阶梯的最后一段不一样了。岩石不再干燥，脚下变得更暗、更凉，空气里有湿石头的气味，水面反射上来的光在底部一个低矮洞穴的顶上投下晃动的波纹。

The last stretch of the staircase is different. The rock stops being dry. It goes darker and cooler under her feet, and there is a smell of wet stone, and the light coming up off the water throws moving lines across the ceiling of a low cave at the bottom.

就在那里，与海面齐平，咸水不过几步之遥，岩石里却渗出了淡水。波莉把喙凑过去。水又凉又甜，完全不合常理，而它正是她头顶上这一切存在的理由。

And there, at the level of the sea, with salt water a few steps away, fresh water is coming out of the rock. Polly puts her beak to it. It is cold and it is sweet and it is entirely wrong, and it is the reason for everything above her.

道理是这样的：崖顶的土地是石灰岩，雨水落在石灰岩上，不像落在别的岩石上那样流走，而是往下渗。它找到裂缝，把裂缝撑宽，在石头内部一路向下。又因为淡水比海水轻，它能一直流到悬崖底部，赶在大海与它混合之前，到达水线时依然是淡的。

It works like this. The land on top of the cliff is limestone, and rain does not run off limestone the way it runs off other rock. It sinks. It finds cracks and widens them and travels down through the inside of the stone, and because fresh water is lighter than sea water it can come all the way to the bottom of the cliff and arrive still fresh, right at the water line, before the sea gets a chance to mix with it.

这样一来，这道阶梯就说得通了。不是士兵，也不是一夜之间。多半是修士，年复一年，耐心地向下凿，朝着一眼他们知道就在那里的泉水。因为一座建在悬崖上的城，可以有城墙、有塔楼、能俯瞰整个海峡，却仍会因为缺一口水而失去一切。

So now the staircase makes sense. Not soldiers, and not one night. Monks, most likely, cutting patiently downward over years toward a spring they knew was there, because a town on top of a cliff can have walls and towers and a view over the whole strait and still lose everything for the want of a drink of water.