heat as the sun lavishes abroad to ungrateful space in the tenth part of every single second. During the few minutes that the reader has been occupied over these lines, a quantity of heat which is many thousands of times as great as, the heat which could be produced by the ignition of all the coal in every coal-pit in the globe has been dispersed and totally lost to the sun. But we have still one further conception to introduce before we shall have fully grasped the significance of the sun's extravagance in the matter of heat. As the sun shines to-day on this earth, so it shone yesterday, so it shone a hundred years ago, a thousand years ago; so it shone in the earliest dawn of history; so it shone during those still remoter periods when great animals flourished which have now vanished forever; so it shone during that remarkable period in earth's history when the great coal forests flourished; so it shone in those remote ages many millions of years ago when life began to dawn on an earth which was still young. There is every reason to believe that throughout these illimitable periods which the imagination strives in vain to realize, the sun has dispensed its radiant treasures of light and warmth with just the same prodigality as that which now characterizes it. We all know the consequences of wanton extravagance. We know it spells bankruptcy and ruin. The expenditure of heat by the sun is the most magnificent extravagance of which human knowledge gives us any conception. How have the consequences of such awful prodigality been hitherto averted? How is it that the sun is still able to draw on its heat reserves from second to second, from century to century, from eon to eon, ever squandering two thousand million times as much heat as that which genially warms our temperate regions, as that which draws forth the exuberant vegetation of the tropics, or which rages in the Desert of Sahara? This is indeed a great problem. It was Helmholtz who discovered that the continual maintenance of the sun's temperature is due to the fact that the sun is neither solid nor liquid, but is to a great extent gaseous. His theory of the subject has gained universal acceptance. Those who have taken the trouble to become acquainted with it are compelled to admit that the doctrine set forth by this great philosopher embodies a profound truth. [Illustration: A TYPICAL SUN-SPOT. By permission of Longmans, Green & Co., from "Old and New Astronomy," by Richard A. Proctor.]