How do submarines go up and down in the water?
The simple answer is: they make themselves heavier in order to submerge (sink) and lighter to surface (float). The mechanisms they use to accomplish this are called ballast tanks.When a submarine on the surface of the ocean wants to submerge, the crew will remotely open the ballast tank air vents, allowing them to flood with seawater. The submarine becomes denser (heavier) in relation to the ocean, and submerges. When the time comes to surface, the vents are closed, and the seawater in the ballast tanks is expelled with pressurized air. The vessel becomes lighter (less dense) relative to the surrounding water, and thus buoyant. So it rises.Interestingly, once submerged, the boat can travel deeper or shallower without taking on or expelling water. It does this in much the same way that an airplane flies, by maintaining headway through the water, and controlling depth with its planes (the stubby “wings” sticking out on either side of the sail or tower).Also, in emergency situations, the ballast tanks can be emptied very rapidly, by blowing the water out with very high pressure (~4000+psi) air. When you see photos of submarines “flying” out of the water, they are performing an “emergency ballast tank blow” procedure, which allows them to surface very quickly.Hope this answers your question,MM2 (SS)
How do buildings in Venice manage to stand despite being surrounded by water on all sides and being constructed over water?
The Grand Canyon is 17million years old. Venice (in its current form) is only 600 years old. It was then they diverted the rivers to surround the city, using the flowing water to deepen the lagoon for military defense. Given enough time, Venice will be washed away :)In the meantime, the buildings stay up because they're not built directly on the mud. They're built on closely spaced wooden piles sunk through the mud and into a denser layer of clay beneath. Picture from an awesome blog post here: Chapter 6: Venice: Climatic variability, local construction methods The wood is fully submerged, and without oxygen won't rot (there are bronze age piers in the river thames still!) and even if a small amount of the mud is washed away during a bad storm/tide the piers will still carry all of the weight into the deep clay meters below. Erosion is a tricky question. The Venetian lagoon is flushed by tides (flowing in and out) and also by the rivers carrying slit down. You'll have material moving back and forth with each tide, seasonal changes as more is carried down or carried away and plenty of complex local movement where material literally crosses the street but there's no net volume change over the region.Venice has been subsiding recently, but this was due to artisan wells draining water out of the aquifer underneath the city (underneath the clay). In this context, in reality the ground was sinking but not eroding, with the city just following it down.