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Some of the best moments in World War II movies occur when, during a battle at sea, a periscope pokes up from the water, surveying the ocean terrain.
二战题材的电影中一些可赞的片段是，海上作战的时候，一个潜望镜从水中穿出，用以测量海洋地形。
Cut to a submarine gliding through the depths. Torpedoes away, and the enemy battleship is doomed. How do these amazing vessels rise and sink so quickly to specific depths?
镜头切换到潜水艇潜到水下。然后发射鱼雷，敌军战队注定会失败。这些神奇的军舰是如何实现快速上升下沉到特定的深度?
Although submarines are sophisticated machines, their ability to rise and sink is based on the elementary principle of buoyant force.
尽管潜艇是很复杂的机器，但它们上升下沉所依据的却是浮力的基本原理。
Say you fill your bathtub with water and put a rubber gym ball on the surface of the water.
将浴缸填满水然后在水面上放一个橡胶健身球。
It floats because the air-filled ball is less dense than the surrounding water.
它会在水面上漂着因为充气球的密度小于水的密度。
But poke a hole in the ball and as water displaces the air, the overall density increases until the ball is heavy enough to sink to the bottom.
但在球上戳一个洞，用水代替健身球中的空气，其整体密度会变大，一直到该球重的足以落到水的底部。
译文来自可可英语
