To test by an independent process the accuracy of the unit of my pressure gauge, on which the estimated corrections for the “Challenger” deep-sea thermometers depend, it was arranged that H.M.S. “Triton” should visit during the autumn a region in which soundings of at least a mile and a half could be had. A set of manometers, filled with pure water, and recording by the washing away of part of a very thin film of silver, were employed. They were all previously tested, up to about 2½ tons weight per square inch, in my large apparatus. As I was otherwise engaged, Professor Chrystal and Mr Murray kindly undertook the deep-sea observations; and I have recently begun the work of reducing them.