=194. The Mechanical Equivalent of Heat.=--While watching workmen bore holes in cannon, Count Rumford, 1753-1814, noticed with much interest the large amount of heat produced in the process. He observed that the heat developed seemed to have some relation to the work done upon the drill in boring the holes. Later experiments performed by many men indicated that a definite relation exists between the heat produced by friction and the amount of work done in overcoming the friction. This discovery indicates that in some way heat is related to energy and that heat is probably a form of energy. Later experiments have confirmed this idea, and it is now considered well established that _heat is a form of energy_. Many attempts have been made to discover the relation between the units of heat energy and the units of mechanical energy. To illustrate one method employed, suppose one measures a given _length_ in inches and in centimeters; on dividing one result by the other, it will be found that a certain relation exists between the two sets of measurements, and that in every case that 1 in. equals 2.54 cm. Similarly, when the same amount of _energy_ is measured both in heat units and in work units a constant relation is always found between the units employed. _One B.T.U. is found equivalent to 778 ft.-lbs. 1 calorie being equivalent to 42,700 g. cm. (427 g. m.)._ This relation is called the _mechanical equivalent of heat_, or in other words it represents the number of work units equivalent to one heat unit. [Illustration: FIG. 162.--Apparatus for determining the mechanical equivalent of heat.] [Illustration: William Gilbert (1540-1603), "Father of magnetic philosophy." Especially noted for his experiments and discoveries in magnetism; first to use the word "electricity." First man to practically emphasize experimental science. DR. WILLIAM GILBERT (Popular Science Monthly)] [Illustration: James Prescott Joule (1818-1889), England, determined the mechanical equivalent of heat; discovered the relation between an electric current and the heat produced; first proved experimentally the identity of various forms of energy. JAMES PRESCOTT JOULE (Popular Science Monthly)]