Physicists have succeeded in solving the difficult problem, which consists of ascertaining how many kilogrammeters may be produced by a _calorie_, transformed into mechanical labour. The works of Mayer, Joule, Helmholtz, Hirn, Regnault, &c., establish that a calorie is equivalent to 425 kilogrammeters, that is to say that the quantity of heat requisite to raise the temperature of a kilogram of water to 1 degree centigrade produces a mechanical action represented by the elevation of a weight of 425 kilograms 1 yard (_métre_) in height in the space of a sound. 425 kilograms are called the _mechanical equivalent of heat_.
With this information at our service, we are enabled to calculate in units of mechanical force the work done by solar heat, by transforming itself into mechanical force. And, if we calculate the total heat of the sun diffused over the earth, during a given time, we can calculate the sum of the forces which all this distributed heat would develop on the surface of the earth, if it were all employed in mechanical labour. In one year every square yard of the surface of the earth receives 2,318,157 calories, that is to say, more than 23,000,000,000,000 of calories to each space of 2 acres, 1 rood, 35 perches.[5]
To understand the intensity of this force, we must conceive a steam engine, which, instead of working at 200 or 300 horse-power, like the engines of our larger steamers, should work at 4,163 horse-power. And this, we must bear in mind, refers only to the small space above mentioned. If we calculate the entire surface of the earth, we arrive at the astounding total of 217,316,000,000,000 horse-power. In order to conceive such a force, we must picture to ourselves 543,000,000,000,000 steam engines each working without relaxation day and night, at 400 horse-power. That is the amount of work which the heat of the sun does for our globe alone.
The physical and mechanical actions which take place on our planet, vegetation, the phenomena of animal life, industrial and agricultural operations absorb only a very small quantity of this enormous mass of forces. Professor Tyndall says on this subject, in the book we have already quoted:--
"Look at the integrated energy of our world--the stored power of our coal-fields; our winds and rivers; our fleets, armies, and guns. What are they? they are all generated by a portion of the sun's energy which does not amount to 1/2300000000th of the whole. This, in fact, is the entire fraction of the sun's force intercepted by the earth, and in reality we convert but a small fraction of this fraction into mechanical energy. Multiplying all our powers by millions of millions, we do not reach the sun's expenditure."--p. 433.