MESSRS EDITORS—In your issue of the 31st ult., you briefly allude to a paper by George Rennie, of London, on this interesting subject. During the exhibition at the New York Crystal Palace in 1853, I had many opportunities to notice the effect of friction on the temperature of water. You and many of your readers will remember among the pumps then on exhibition two of "Gwynn's Centrifugal Pumps;" one the large fountain pump, in the east nave, with a capacity for elevating 7,000 gallons of water per minute ; and the other, a smaller one in the machine arcade, with a capacity of about 300 gallons per minute. Both these pumps were arranged to work the same water over till it was best to change it, and in both the effect in raising the temperature was very apparent. The suction pipe of the small pump was four inches in diameter, while the discharge was but 2^- inches, and yet at an elevation of but five or six feet, 400 gallons per minute or 26,000 gallons per hour were constantly driven through this small orifice when the pump was in motion. This motion in an atmosphere not higher than 40 or 50 degrees would in a few hours bring the water to blood heat, and in the summer weather, if I may judge by comparison and my own feelings, I have frequently noticed the temperature of water so heated as high as 150 The larger pump was erected with a view to the "Delights of a Gushing Fountain," and the cooling effect of water in motion, and when a fresh supply was introduced from the Croton, we in a measure realized our hopes ; but, after a short agitation, the glow was changed by a scattering damp that made it impracticable to keep it long in motion. The laborious duties of my position prevented me from noting for publication at the time this, as well as many other interesting phenomena, which should more attract the investigation of philosophic minds. I am glad such men as George Rennie have directed their especial attention to this matter, and I hope we shall learn the true cause and source of the heat evolved. My own opinion has been that the sensible heat apparent is more the result of the sudden compression of the particles or bubbles of air constantly carried into the water, and by its force possibly reduced momentarily to one-half or even one-fourth of its natural bulk, thus for the moment having its heat doubled or quadrupled above the temperature of the water, and, of course, giving off a portion to the water surrounding it. The air bubbles do not fully regain their natural size or bulk till they are liberated from the water and do not carry out as much heat as they carried in, and to this I attribute the cooling and grateful effects of "babbling brooks" and the "rushing waterfall," so long as the supply comes fresh from the fountain. Please let us have more of your own philosophy on this subject, and induce some one who has leisure to try further experiments by keeping the surrounding atmosphere within one or two degrees of the increased temperature of the water, and also to try the agitation in a vacuum, and let us know whether pure water can be heated by friction. JOSEPH E. HOLMES. Newark, Ohio, Nov., 1857. [The opinion hitherto in vogue among philosophers is opposed to an increased heat being produced in liquids by their own friction, and also to an increased heat being produced by the friction of a current of air or gas upon a liquid or solid. It is well known that water coutains a portion of atmospheric air, and by compressing it, heat will be changed from low to high intensity. As the discharge pipe of the pump to which Mr. Holmes alludes was much smaller than the suction, the air in the water must have been compressed, as he suggests, thus developing increased sensible heat, a part of which was left in the water, even when the air escaped, because the water has nearly five times a greater capacity for heat than air, and therefore parts with its caloric more slowly. Probably the heat generated by the friction of the solid parts of the pump was also communicated to the water, and tended to elevate its temperature. Count Rumford, by boring a cylinder of cast iron, raised the temperature of several pounds of water to the boiling point.