IN NATURE of August 3, p. 316, is given a letter from Rear-Admiral Sir William Wharton, in which he states that he is diametrically opposed to my opinion about the double currents in the Straits. He says that “Admiral Makaroff considers that difference of density of the water is the primary, and, indeed I gather he thinks, the only cause of these opposing currents; but he brings no evidence beyond theoretical considerations in support of his belief”; further, in his letter, Admiral Wharton refers particularly to the double current of the Bosporus, of which I spoke in my lecture at the Royal Society of Edinburgh. I cannot leave unnoticed remarks from so distinguished a hydrographer, who, during his long work, has contributed so much to the advance of science. My researches about the Bosporus are published only in Russian, in a book named “On exchange of water between Black Sea and Mediterranean” (St. Petersburg, 1885). Should Admiral Wharton know my language, he would easily come to the conclusion that my opinion about double currents in the Bosporus are based upon the observations made in 1881 and 1882. I then invented an instrument for measuring the current at different depths, and gave the name of “fluctometer” to it. The instrument consists of a propeller revolving on a horizontal spindle. A bell is attached to the propeller, and at every revolution of the propeller it strikes twice. As water is a very good conductor of sound, the number of revolutions could be counted through the bottom of the ship (provided the ship is not sheeted with wood) at all depths to which the instrument was lowered (40 fathoms). I used to anchor in the middle of the Bosporus for a couple of days at a time, and make a series of observations every two hours. In order to obtain more detailed data, I used to take the samples of water from the same depth to which the fluctometer was lowered. Twice I used to go along the Bosporus from the Black Sea to the Marmora Sea in order to learn in what depth is the limit of two currents. In volume xxii. of the Proceedings of the Royal Society of Edinburgh, Plate I. shows a position of the limit of two currents, mean velocity of both currents, and specific gravity of water. In Plate II. is given a sketch of my “fluctometer.”