This first natural mode of liberating energy within their own bodies does not satisfy the growing needs and aims of men. Such a mode is "labor," which becomes at times painful and distasteful. In the earliest societies known, some sorts of domestic animals are found supplementing man's efforts and acting upon the material world to alter it for man. The dog joining in the chase guards his master's safety, and helps to bear his burdens. The draft-beast in the field turns the heavy soil, and aids in the final harvest. The trained elephant does the work of twenty men piling logs, loading ships, or carrying burdens. Man further increases his control over the material world by making other men do his bidding. Domestic slavery, where wife or child serves the father of the family, or chattel slavery, where the vanquished toils for the victor, are all but universal in early communities. Such a method of increasing one's control over the forces of the world requires only superior strength, no special intelligence in mechanics, and is thus one of the first crude devices in a primitive civilization. Fuel has been, up to the present time, perhaps the most important source of energy. Fire in the hands of savage man gave him dominion over the forests and over the metals. In this age of steam the liberation of the energy of the sun, stored up in coal in ages past, is still the indispensable condition of our developed industry. [Sidenote: By the energy in wind and flowing water] 4. _The greatest and most exhaustless reservoirs of power for man's use are in wind and water._ While the supply of fuel is being used at a progressive rate and will soon approach exhaustion, there are elsewhere exhaustless stores of energy awaiting man's command. To make use of the wind for sailing a boat, only the simplest arrangements are needed; a windmill fixed at one place requires more ingenuity and machinery. The energy of the wind is derived from the sun and will last until the sun loses its heat. If some means can be found for equalizing the flow and for storing the power of the wind, it may yet become a great agency of industry. The force of falling water, long used in a petty way by the old water-mills, is just beginning to be employed on a large scale at such points as Niagara. Where fuel is high, as on the Pacific coast, wave motors have been successfully used in a small way, but wave motion is too irregular to serve well the needs for power. But the constant motion of the tides offers, at some favored points, a source of power that will remain as long as the earth revolves upon its axis. [Sidenote: By the intelligent utilization of all these agencies]