"The smooth surface of the sea reflects heat better than the rough land: for this reason, a larger proportion of the heat which falls upon the sea is not absorbed, but reflected and lost, so far as the temperature of this world is concerned. Water is also a very poor conductor of heat, and has withal a very high specific heat. For these reasons the sea receives and parts with heat more slowly than the land, and its absorption or radiation causes a smaller variation of temperature. The result is, therefore, that the sea is cooler in summer and warmer in winter than the land, and the average ocean temperature is lower than the mean continental temperature. The land receives heat more readily and parts with it more rapidly; the fluctuations of temperature must therefore be greater. Hence, the interiors of the continents have much greater extremes of temperature than the sea-board. But of the influence of water in equalizing temperature I shall have occasion to speak again more at length, and will pass it by for the present. What effect, Peter, has the unevenness of the earth's surface upon temperature?" "The higher we ascend upon mountains, the colder we find it." "That is, Peter, the greater the elevation of any place or country above the sea level, the lower the temperature. Almost the whole surface of the earth is an alternation of mountain and hill, valley and plain. One continent has a very much greater mean elevation than another. One region or tract of country lies sloping toward the sun, another is inclined from it. The effect in the one case is the same as if the sun were brought more nearly overhead; in the other case, the sun is depressed toward the horizon. It is all the same as if the region of country were brought nearer the equator or removed farther from it. The effects of the curvature of the earth are obviated or exaggerated. Do clouds tend to produce inequalities of temperature?" "I think they must do this," answered Samuel. "Clouds cover one portion of the earth's surface and shut out the heat of the sun, while other portions are well exposed to the sun's rays." "That is right, Samuel. Does any one think of another cause of inequality of temperature?" There was a pause. Then Mr. Hume answered: "Considering the unmeasured cycles of the past, the gradual cooling of the earth has brought a great change of temperature." "And this change," continued Mr. Wilton, "has been very important for the welfare of the human race. At the present temperature of the earth, the coal-beds, so necessary for the culture and progress of the race, could hardly have been formed, and at the temperature of the carboniferous periods, when the coal-beds were deposited, the human race could with difficulty have survived. The high temperature required to prepare the earth for man is now no longer needed, but would prove destructive. And this great change of temperature was doubtless caused by the cooling of the earth.