203. We proceed next to those varieties of available energy which represent motion, the chief of which are air in motion and water in motion. It is owing to the former that the mariner spreads his sail, and carries his vessel from one part of the earth’s surface to another, and it is likewise owing to the same influence that the windmill grinds our corn. Again, water in motion is used perhaps even more frequently than air in motion as a source of motive power. Both these varieties of energy are due without doubt to the heating effect of the sun’s rays. We may, therefore, affirm that with the exception of the totally insignificant supply of native sulphur, &c., and the small number of tidal mills which may be in operation, all our available energy is due to the sun. _The Sun--a Source of High Temperature Heat._ 204. Let us, therefore, now for a moment direct our attention to that most wonderful source of energy, the Sun. We have here a vast reservoir of high temperature heat; now, this is a kind of superior energy which has always been in much request. Numberless attempts have been made to construct a perpetual light, just as similar attempts have been made to construct a perpetual motion, with this difference, that a perpetual light was supposed to result from magical powers, while a perpetual motion was attributed to mechanical skill. Sir Walter Scott alludes to this belief in his description of the grave of Michael Scott, which is made to contain a perpetual light. Thus the Monk who buried the wizard tells William of Deloraine-- “Lo, Warrior! now the Cross of Red Points to the Grave of the mighty dead; Within it burns a wondrous light, To chase the spirits that love the night. That lamp shall burn unquenchably Until the eternal doom shall be.” And again, when the tomb was opened, we read-- “I would you had been there to see How the light broke forth so gloriously, Stream’d upward to the chancel roof, And through the galleries far aloof! No earthly flame blazed e’er so bright.” No earthly flame--there the poet was right--certainly not of this earth, where light and all other forms of superior energy are essentially evanescent. _A Perpetual Light Impossible._ 205. In truth, our readers will at once perceive that a perpetual light is only another name for a perpetual motion, because we can always derive visible energy out of high temperature heat--indeed, we do so every day in our steam engines.