199. The energy of food is analogous to that of fuel, and serves similar purposes. For just as fuel may be used either for producing heat or for doing work, so food has a twofold office to perform. In the first place, by its gradual oxidation, it keeps up the temperature of the body; and in the next place it is used as a source of energy, on which to draw for the performance of work. Thus a man or a horse that works a great deal requires to eat more food than if he does not work at all. Thus, also, a prisoner condemned to hard labour requires a better diet than one who does not work, and a soldier during the fatigues of war finds it necessary to eat more than during a time of peace. Our food may be either of animal or vegetable origin--if it be the latter, it is immediately derived, like fuel, from the energy of the sun’s rays; but if it be the former, the only difference is that it has passed through the body of an animal before coming to us: the animal has eaten grass, and we have eaten the animal. In fact, we make use of the animal not only as a variety of nutritious food, but also to enable us indirectly to utilize those vegetable products, such as grasses, which we could not make use of directly with our present digestive organs. _Head of Water._ 200. The energy of a head of water, like that of fuel and food, is brought about by the sun’s rays. For the sun vaporizes the water, which, condensed again in upland districts, becomes available as a head of water. There is, however, the difference that fuel and food are due to the actinic power of the sun’s rays, while the evaporation and condensation of water are caused rather by their heating effect. _Tidal Energy._ 201. The energy derived from the tides has, however, a different origin. In Art. 133 we have endeavoured to show how the moon acts upon the fluid portions of our globe, the result of this action being a very gradual stoppage of the energy of rotation of the earth. It is, therefore, to this motion of rotation that we must look as the origin of any available energy derived from tidal mills. _Native Sulphur, &c._ 202. The last variety of available energy of position in our list is that implied in native sulphur, native iron, &c. It has been remarked by Professor Tait, to whom this method of reviewing our forces is due, that this may be the primeval form of energy, and that the interior of the earth may, as far as we know, be wholly composed of matter in its uncombined form. As a source of available energy it is, however, of no practical importance. _Air and Water in Motion._