The measuring units of mass, length, time, and temperature are fundamental, others like velocity, acceleration, power, and area are referred to them. For that reason the latter are called derived units. Many of these are important and call for accurate determinations. One of the first achievements of the century was the establishment of the doctrine of the conservation of energy. Francis Bacon had suggested that motion is a phenomenon of heat, and Newton had divined the principle of the conservation of energy, but it was Benjamin Thompson, Count Rumford, who discovered the nature of friction and made the first estimate of the mechanical equivalent of heat. Sir Humphry Davy showed that two pieces of ice could be melted by simply rubbing them together, in a vacuum. But he failed to draw the great inference that this experiment warranted. If he had observed that the heat could not have been supplied by the ice because ice is an absorber of heat, he would have anticipated the great work done by James P. Joule, an English physicist, who published the results of many experiments carried out by him prior to 1843. His task was to find the exact mechanical equivalent of heat. His best results were secured by dropping a mass of lead from a measured height and using the energy generated during the descent to operate a revolving paddle in a dish of measured water. Delicate thermometers recorded the increase of temperature in the water and showed that the descent of 424 grams of lead through a distance of one meter, or one gram of lead through 424 meters, generated sufficient heat to raise one gram of water one degree centigrade (1° C.). Otherwise expressed, a fall of 772 lbs. of lead through a distance of 1 foot, or 1 lb. of lead through 772 feet, raises the temperature of 1 lb. of water one degree Fahrenheit (1° F.). These 772 foot-pounds, or 424 gram-meters, represent the mechanical equivalent of heat upon which so many important theories have been based. But Joule's equivalent was determined for common air temperatures whereas the specific heat of water increases with the temperature so that the value of the equivalent rises with increased temperatures. Osborne Reynolds, in 1897, found the mean equivalent for temperatures between the freezing and boiling points to be 777 foot-pounds.