Page:Elementary Text-book of Physics (Anthony, 1897).djvu/214

200 rise of temperature of 35° per hour; while in Joule's experiments the rise of temperature per hour was less than 1°. These experiments give, for the mechanical equivalent of one calorie at 5°, 429.8 kilogram-metres; at 30°, 436.4 kilogram-metres.

Several other methods have been employed for determining the mechanical equivalent. The concordance of the results by all these methods is sufficient to warrant the statement that the expenditure of a given amount of mechanical energy always produces the same amount of heat.

An experiment to determine the mechanical work done by the expenditure of a known quantity of heat was executed by Hirn. By the help of Regnault's measurements of the heat of vaporization Hirn was able to calculate the amount of heat which entered the cylinder, during the operation of a steam-engine, with the steam from the boiler, and by direct measurements he determined the amount of heat which left the cylinder during the operation of the engine and entered the condenser. So long as the engine was running without doing any external work, he found that these amounts of heat were appreciably equal; when the engine was made to do work, less heat passed from the cylinder into the condenser than had entered it from the boiler. A comparison of the amount of heat lost with the work done by the engine showed the same ratio between heat and work as that determined by Joule. Hirn's experiments were on so large a scale and the sources of error and the difficiilties connected with the experiments were so numerous, that the number obtained by him for the mechanical equivalent of heat is of no great value. His experiments are, however, of very great interest because, while the experiments of Joule and of all the others who have worked on the problem prove the convertibility of work into heat, those of Hirn alone have proved the converse convertibility of heat into work.