On page 323 of the present volume of the SCIENTIFIC AMERICAN, we gave a description- of this instrument, and a few days ago we had the pleasure of seeing it practically, tested in the presence of Professor A. D. Bache. The tests were two observations which had been taken with this instrument by Captain Foster, on board the Marion, from New York to Charleston. The Captain also took observations with his sextant, and by that means obtained the latitude. This being known, the question was whether the helypsometer would give the same latitude, and on opening it (for it had been sealed up), and the necessary simple observations and calculations made, the latitude was found to differ only from t wo to four minutes of a degree from that formed by the Captain's sextant. This is as near as any two observers with sextants or quadrants on the deck of the same vessel could make the observation, and is a much nearer approximation to exactitude than was ever obtained by any instrument that has been used to obtain latitude when the natural horizon was obscured. The instrument will prove very valuable in the foggy regions of Newfoundland and the coasts of Great Britain, and does much credit to the inventor, Mr. John Oakes, of this city, who has secured it by Letters Patent in this country and in Europe through the Scientific American Patent Agency.