Pointer of the spring balance, 0.05 N divisionsOverflow can filled to the spoutMeasuring cylinder collecting what is pushed outWeigh the solid in air, then lower it into an overflow can full to the spout. The balance reads less: the water pushes it up (upthrust), and the water pushed out into the cylinder weighs exactly as much as the loss. The push comes from the pressure p = hρg, bigger on the deeper bottom face than on the top face. Fully under, going deeper changes nothing. Strongly salty water is denser, so the same volume weighs more and the loss is bigger. The stone weighs about the same as the iron but has three times its volume, so it loses three times as much. g = 9.8 N/kg.