Why should the emitted neutrons be slowed down in sustainable chain reaction

your answer would be fusion

in simple terms, the principle states that the buoyancy force on an object is equal to the weight of the fluid displaced by the object, or the density of the fluid multiplied by the submerged volume times the gravitational acceleration, g. this is also known as upthrust.

layers of the earth affect the waves and these effects are seen on instruiments such as seimometers