It is required to subtract the gravity of the bucket from the total gravity of the bucket and water, so d is to measure the gravity of the empty bucket;

(1)① Measure the gravity of the metal block in the air, that is, f1;

(2) The buoyancy of an object is equal to gravity minus the indicated value of the spring dynamometer when the object is immersed in water, that is, f float = f1-f2;

③ Given the weights of buckets and water and the weight of empty buckets, the weight of boiled water is: g row = F3-F4;

④ Finally, the relation F float =G row holds, that is, F 1-F2=F3-F4, and the famous Archimedes principle can be obtained.

(2) The object floats on the water, and only a part of it is immersed in the water. If we want to measure the buoyancy of floating wood blocks, we should let them float on the water instead of immersing them in the water.

So the answer is: the gravity of an empty bucket;

( 1)f 1； f 1-F2； F3-F4； f 1-F2 = F3-F4； Archimedes;

(2)b； Blocks of wood floated on the water.