Which device?
More:
The modern submarine has hundreds of devices on-board used for anything imaginable. Most devices have back-ups. The nuclear reactor has a back-up diesel. There are at least 2 primary propulsion plants. Even the distiller has a smaller back-up. The distiller is the second most important device to an underway crew. Without it there is not enough water for showers. Moral would plummet and no one likes a smelly shipmate.
A standard viewing device used on a submarine is a periscope.
A periscope.
Ballast tank
ASDIC
Sonar is used to detect other vessels or ships/submarines through a tube-like device> Andrea
This sounds like it's probably the sonar system of a submarine.
sonar is a device for detecting objects underwater by emitting sound waves and analyzing the echoes that bounce back. It is commonly used in navigation, fishing, and underwater mapping.
By an electronic device called 'sonar' .
During WW2, all the major countries involved used submarines.
They were used by submarines to navigate when not in open waters, They were also used by submarines and anti-submarine ships to locate enemy submarines.
Submarines todaee are used for underwater exploration,funn,new life underwater.
Nuclear energy is used to power most modern submarines