No. 11
Yet when the hordes to happy nations rise, And earth by culture warms the genial skies,
Book II. Line 119.
Without entering into any discussion on the theory of heat and cold (a point not yet settled in our academies) I would just observe, in vindication of the expression in the text, that some solid matter, such for instance as the surface of the earth, seems absolutely necessary to the production of heat. At least it must be a matter more compact than that of the sun's rays; and perhaps its power of producing heat is in proportion to its solidity.
The warmth communicated to the atmosphere is doubtless produced by the combined causes of the earth and the sun; but the agency of the former is probably more powerful in this operation than that of the latter, and its presence more indispensable. For masses of matter will produce heat by friction, without the aid of the sun; but no experiment has yet proved that the rays of the sun are capable of producing heat without the aid of other and more solid matter. The air is temperate in those cavities of the earth where the sun is the most effectually excluded; whereas the coldest regions yet known to us are the tops of the Andes, where the sun's rays have the most direct operation, being the most vertical and the least obstructed by vapors. Those regions are deprived of heat by being so far removed from the broad surface of the earth; a body that appears requisite to warm the surrounding atmosphere by its cooperation with the action of the sun.
From these principles we may conclude that cultivation, in a woody country, tends to warm the atmosphere and ameliorate a cold climate; as, by removing the forests and marshes, it opens the earth to the sun, and allows them to act in conjunction upon the air.
According to the descriptions given of the middle parts of Europe by Cesar and Tacitus, it appears that those countries were much colder in their days than they are at present; cultivation seems to have softened that climate to a great degree. The same effect begins to be perceived in North America. Possibly it may in time become as apparent as the present difference in the temperature of the two continents.