Tracer Bullets were invented so the attacking force could see whether they were hitting the attacking force or not. They were invented by the British so that they could see at night wether they were hitting the enemies or not. A disadvantage though is that the enemy could see the bullets and where they were coming from. They let loose oil from the sides of them and lit the oil with fire that came from the bullet. When they shot them they would look at where they hit and see the enemy.
Depth Charges were invented by the British too! They were used to block the enemies torpedo shots at them. The Depth Charges had a special thing in them that calculated how deep it was and at a certain depth, it exploded giving it the name Depth Charge. The British saved many ships that way and saved their navy. This gave them a tactical advantage in the sea.