During my long investigations of the “Pressure-Errors of the ‘Challenger’ Thermometers,” but more especially two years ago (Proceedings, May 1881), I was led to suspect a lowering of the maximum density point of water by pressure. For I found that the change of temperature of water increased faster than in direct proportion to the sudden change of pressure which produced it. These experiments were, at my request, more fully carried out by Messrs Marshall, Smith, and Omond (Proceedings, July 1882). Their result was (approximately) a lowering of the maximum density point by 5° C. for 1 ton-weight of pressure per square inch (roughly speaking, about 150 atmospheres). In a note appended to their paper I deduced from their experimental data a lowering of 3° 6 C, and from my own a lowering of about 3° C. for the same pressure.