AN ingenious instrument has lately been described by Dr. J. W. Moll (Archives Neerlandaises, i. xviii.) under the name of the Potometer. It is a modification of Sachs' apparatus for determining the amount of water which a cut branch absorbs in a given time. I have been for some years in the habit of using a form of Sachs' instrument, differing in principle from Moll's apparatus, but resembling it in being especially adapted for making observations at short intervals of time. As the subject of transmission of water through wood is now attracting attention among physiologists, it seems worth while to describe my instrument.