of the energy now existing in consequence of the rotation will have been reduced to a fourth of its present value.
Next as to the various forms in which sun-heat is received. We have already referred to the mode in which it is captured by growing plants. There is also another indirect method in which the sun-heat is made to provide energy useful to man. The waterfall which turns the mill-wheel is of course really efficient because the water is running down, and it can only run down because it has first been raised up. This raising is accomplished by sunbeams. They beat down on the wide expanse of the great oceans, and the vapour thence arising soars aloft into the heights of the atmosphere where it forms clouds. It is of course the solar energy that has performed this task of lifting, and as the rain descends it becomes collected into the streams and rivers which on their way to the sea are made to turn the waterwheels. In like manner it is of course the action of the sun which sets in motion great volumes of air to form the winds, so that when we employ windmills to grind our corn we are utilising energy diffused from the sun.
It goes without saying that the welfare of the human race is necessarily connected with the continuance of the sun's beneficent action. We have indeed shown that the few other direct or indirect sources of heat which might conceivably be relied upon are in the very nature of things devoid of the necessary permanence. It becomes therefore of the utmost interest to inquire whether the sun's heat can be calculated on indefinitely. Here is indeed a subject which is literally of the most vital importance so far as organic life is concerned. If the sun shall ever cease to shine, then must it be certain that there is a term
