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。

A submarine works in much the same way, with one major exception: a sub can alter its buoyancy by manipulating overall density.

海底工作大致是相同的，但有一点例外：通过操控整体密度，潜水艇能够改变其浮力。

Submarines come equipped with tanks that can be filled with various levels of water and air.

潜艇都配备蓄水池，贮备着不同的水量和空气。

When a sub floats on the surface, its tanks are filled mostly with air, which makes the sub less dense than the surrounding water.

但潜水艇浮在水面的时候，它的蓄水池里大部分都是空气，这样可以使得潜水艇的密度小于水的密度。

When the crew wants to make the sub sink they open valves to allow just enough water to flood the tanks and make the sub sink to a specific depth.

若工作人员想要使潜艇下沉，他们可以打开阀门，让足够多的水流入蓄水池，然后使其下沉到一个特定的深度。

Of course the sub must be able to rise again after filling its tanks with air.

当然将蓄水池充入空气，潜艇会再次上升。

To that end submarines carry tanks of compressed air that can be used to displace water from the tanks and increase buoyancy.

最后潜艇会携带带有压缩空气的蓄水池，用来替换蓄水池中的水，增加浮力。