When the hydrometer is closed its mass cannot be diminished, and it can be increased only by external additions, which in practice must be immersed either in air or in the liquid. The use of submerged weights is attended by so much inconvenience that it has to be avoided; consequently, when the instrument is closed, its mass is increased only by adding weights at the top of the stem. The extent to which such additions can be made depends on the stability of the instrument when floating in the experimental liquid. The instrument (No. 0) which I used exclusively during the voyage of the Challenger weighed, in round numbers, 160 grams, and the greatest weight which had to be added to it was 4·071 grams, which produced no disturbing effect whatever.