時(shí)差N小時(shí):潛水艇 Submarine
二戰(zhàn)題材的電影中一些可贊的片段是，海上作戰(zhàn)的時(shí)候，一個(gè)潛望鏡從水中穿出，用以測量海洋地形。
Cut to a submarine gliding through the depths. Torpedoes away, and the enemy battleship isdoomed. How do these amazing vessels rise and sink so quickly to specific depths?
鏡頭切換到潛水艇潛到水下。然后發(fā)射魚雷，敵軍戰(zhàn)隊(duì)注定會失敗。這些神奇的軍艦是如何實(shí)現(xiàn)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 theelementary principle of buoyant force.
盡管潛艇是很復(fù)雜的機(jī)器，但它們上升下沉所依據(jù)的卻是浮力的基本原理。
Say you fill your bathtub with water and put a rubber gym ball on the surface of the water.
將浴缸填滿水然后在水面上放一個(gè)橡膠健身球。
It floats because the air-filled ball is less dense than the surrounding water.
它會在水面上漂著因?yàn)槌錃馇虻拿芏刃∮谒拿芏取?/p>
But poke a hole in the ball and as water displaces the air, the overall density increases untilthe ball is heavy enough to sink to the bottom.
但在球上戳一個(gè)洞，用水代替健身球中的空氣，其整體密度會變大，一直到該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 itsbuoyancy by manipulating overall density.
海底工作大致是相同的，但有一點(diǎn)例外：通過操控整體密度，潛水艇能夠改變其浮力。
Submarines come equipped with tanks that can be filled with various levels of water and air.
潛艇都配備蓄水池，貯備著不同的水量和空氣。
When a sub floats on the surface, its tanks are filled mostly with air, which makes the sub lessdense than the surrounding water.
但潛水艇浮在水面的時(shí)候，它的蓄水池里大部分都是空氣，這樣可以使得潛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 toflood the tanks and make the sub sink to a specific depth.
若工作人員想要使?jié)撏鲁?，他們可以打開閥門，讓足夠多的水流入蓄水池，然后使其下沉到一個(gè)特定的深度。
Of course the sub must be able to rise again after filling its tanks with air.
當(dāng)然將蓄水池充入空氣，潛艇會再次上升。
To that end submarines carry tanks of compressed air that can be used to displace water fromthe tanks and increase buoyancy.
最后潛艇會攜帶帶有壓縮空氣的蓄水池，用來替換蓄水池中的水，增加浮力。
