The mechanical force which the heat of the sun represents has been calculated, and the numbers thus ascertained are curious. In order to understand how a heat agent can be expressed by figures of mechanical force, we must have a general idea of that theory which constitutes the most valuable creation of natural philosophy in our day; we allude to _the mechanical theory of heat_, or the doctrine of the _mutual transformation of physical forces_. Experience has proved that heat changes, under our eyes, into a mechanical force. See how, by the action of the steam engine, watery vapour becomes cold, and the dispersed heat immediately produces a mechanical force, and you will understand how it is that we maintain that heat transforms itself into force. This being admitted, it is easily explicable that one of those elements may be represented by the others, or that at least we may represent the value of both force and heat by a common unit. This common unit is called a _calorie_, and expresses the quantity of heat requisite to raise the temperature of a kilogram of water one degree. On the other hand, the term _kilogrammeter_ is used to express the quantity of force requisite to raise a kilogram to the height of one yard (_métre_) in a second. 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]