The instrument being closed, its true weight is constant.

Let it be assumed that our experiments are actually made in vacuo, at the sea-level in lat. 45°. In these conditions the standard gram exerts a vertical pressure of 1 gram (true).

We weigh the hydrometer and find its weight to be W grams. We now float it in distilled water contained in a suitable cylinder. In the construction of the hydrometer the internal load has been so adjusted that, when immersed in distilled water of the standard temperature T, which is to remain unaltered during the whole of the experiments, the surface of the water shall cut the stem in some line C, near its junction with the body of the instrument. Then the weight of the water displaced by the hydrometer is exactly W grams.