In this, his Experiment No. 2, the only modification consisted in fitting the steel borer with an air-tight piston, packed with oiled leather, by which any circulation of air from without to the interior was prevented. But in the use of this device the oiled leather itself, by its friction with the sides of the borer, produced considerable heat, so that, to obviate any possible objection as to this point, Rumford had recourse to his third and most celebrated experiment.
In this, the friction cylinder was made to rotate in a water-tight box, which, being filled with water, completely submerged all the heat producing parts. Here, therefore, the only supply of caloric, if any, lay in the water, which itself was to be heated by the friction; for had any caloric been abstracted by the heated water from the ambient air, there would have necessarily been a flow of heat from a cool body to a warmer, which every one admitted to be contrary to experience. The apparatus, therefore, having been arranged, the box was filled with water at the temperature of 60° Fahr., and the machinery put in motion.
With reference to what followed, Rumford remarked:
"The cylinder, revolving at the rate of about thirty-two times in a minute, had been in motion but a short time, when I perceived, by putting my hand into the water and touching the outside of the cylinder, that heat was generated; and it was not long before the water which surrounded the cylinder began to be sensibly warm.
"At the end of one hour I found, by plunging a thermometer into the water in the box (the quantity of which fluid amounted to 18.77 pounds, avoirdupois, or two and a quarter wine-gallons), that its temperature had been raised no less than 47°; being now 107° of Fahrenheit's scale.
"When thirty minutes more had elapsed, or one hour and thirty minutes after the machinery had been put in motion, the heat of the water in the box was 142°.
"At the end of two hours, reckoning from the beginning of the experiment, the temperature of the water was found to be raised to 178°.
"At two hours twenty minutes it was at 200; and at two hours thirty minutes it actually boiled!"The quantity of heat excited and accumulated in this experiment was very considerable; for, not only the water in the box, but also the box itself (which weighed 154 pounds), and the hollow metallic cylinder, and that part of the iron bar which, being situated within the cavity of the box, was immersed in the water, were heated 150° Fahr., namely, from 60° (which was the temperature of the water and of the machinery at the beginning of the experiment) to 210°, the heat of boiling water at Munich."