In order to prevent the honeycombing of the castings by the escaping gas, the cannon were cast in a vertical position with the breech at the bottom of the mould and a short cylinder projecting about two feet beyond the muzzle of the gun, so that any imperfections in the casting would appear in this projecting cylinder. It was on one of these pieces of waste metal, while still attached to the gun, that Rumford conducted his experiments. Having turned the cylinder, he cut away the metal in front of the muzzle until the projecting piece was connected with the gun by a narrow cylindrical neck, 2·2 inches in diameter and 3·8 inches long. The external diameter of the cylinder was 7·75 inches, and its length 9·8 inches, and it was bored to a depth of 7·2 inches, the diameter of the bore being 3·7 inches. The cannon was mounted in the boring-lathe, and a blunt borer pressed by a screw against the bottom of the bore with a force equal to the weight of 10,000 pounds. A small transverse hole was made in the cylinder near its base for the introduction of a thermometer. The cylinder weighed 113·13 pounds, and, with the gun, was turned at the rate of thirty-two revolutions per minute by horse-power. To prevent loss of heat, the cylinder was covered with flannel. After thirty minutes' work, the thermometer, when introduced into the cylinder, showed a temperature of 130° Fahr. The loss of heat during the experiment was estimated from observations of the rate of cooling of the cylinder. The weight of metal abraded was 837 grains, while the amount of heat produced was sufficient to raise nearly five pounds of ice-cold water to the boiling point.
To exclude the action of the air, the cylinder was closed by an air-tight piston, but no change was produced in the result. As the air had access to the metal where it was rubbed by the piston, and Rumford thought this might possibly affect the result, a deal box was constructed, with slits at each end closed by sliding shutters, and so arranged that it could be placed with the boring bar passing through one slit and the narrow neck connecting the cylinder with the gun through the other slit, the sliding shutters, with the help of collars of oiled leather, serving to make the box water-tight. The box was then filled with water and the lid placed on. After turning for an hour the temperature was raised from 60° to 107° Fahr., after an hour and a half it was 142° Fahr., at the end of two hours the temperature was 178° Fahr., at two hours and twenty minutes it was 200° Fahr., and at two hours and thirty minutes it ACTUALLY BOILED!
"It would be difficult to describe the surprise and astonishment expressed in the countenances of the bystanders on seeing so large a quantity of cold water heated and actually made to boil without any fire.
"Though there was, in fact, nothing that could justly be considered as surprising in this event, yet I acknowledge fairly that it afforded me a degree of childish pleasure which, were I ambitious of the reputation of a grave philosopher, I ought most certainly rather to hide than to discover."
Rumford estimated the "total quantity of ice-cold water which, with the heat actually generated by the friction and accumulated in two hours and thirty minutes, might have been heated 180 degrees, or made to boil" at 26·58 pounds, and the rate of production he considered exceeded that of nine wax candles, each consuming ninety-eight grains of wax per hour, while the work of turning the lathe could easily have been performed by one horse. This was the first rough attempt ever made, so far as we know, to determine the mechanical equivalent of heat.
In his reflections on these experiments, Rumford writes:—
It is hardly necessary to add that anything which any insulated body or system of bodies can continue to furnish without limitation cannot possibly be a material substance; and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in the manner the heat was excited and communicated in these experiments, except it be MOTION.
It has been stated that, if Rumford had dissolved in acid the borings and the sawn strips of metal, the capacity for heat of which he determined, and had shown that the heat developed in the solution was the same in the two cases, his chain of argument would have been absolutely complete. Considering the amount of heat produced in the experiments, there are few minds whose conviction would be strengthened by this experiment, and it is only those who look for faultless logic that will refuse to Rumford the credit of having established the dynamical nature of heat.
Davy afterwards showed that two pieces of ice could be melted by being rubbed against one another in a vacuum, but he does not appear to have made as much as he might of the experiment. Mayer calculated the mechanical equivalent of heat from the heat developed in the compression of air, but he assumed, what afterwards was shown by Joule to be nearly true, that the whole of the work done in the compression was converted into heat. It was Joule, however, who first showed that heat and mechanical energy are mutually convertible, so that each may be expressed in terms of the other, a given quantity of heat always corresponding to the same amount of mechanical energy, whatever may be the intermediate stages through which it passes, and that we may therefore define the mechanical equivalent of heat as the number of units of energy which, when entirely converted into heat, will raise unit mass of water one degree from the freezing point.